Dual core waveguide

ABSTRACT

The invention described herein pertains to the structure and formation of dual core waveguide structures and to the formation of optical devices including spot size converters from these dual core waveguide structure for the receiving and routing of optical signals on substrates, interposers, and sub-mount assemblies.

The present patent application is a continuation of U.S. applicationSer. No. 16/582,216, filed on Sep. 25, 2019, now U.S. Pat. No.10,976,497, which is a continuation and claims priority from U.S.application Ser. No. 16/532,770, filed on Aug. 6, 2019, now U.S. Pat.No. 10,976,496, which claims priority from U.S. Provisional application62/803,805, and is related to U.S. Provisional application 62/621,659and U.S. Non-Provisional application Ser. No. 16/036,151, 16/036,179,16/036,208, and 16/036,234, all of which are herein incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates to optoelectronic communication systems,and more particularly to a method for interfacing optoelectroniccircuits with optical fibers and optical devices.

BACKGROUND

In Photonic Integrated Circuits (PICs), various types and quantities ofoptical, electronic, optoelectronic, and electrical functionalities arecommonly combined on a substrate, an interposer or a submount assembly.Interposers are used in the fabrication of optical and electronicdevices as a platform upon which discrete circuit elements are combinedto perform a specific function or set of functions. In opticalassemblies, for example, a set of components such as a sending device, areceiving device, and a waveguide device are combined to create atransceiver. In this example, a laser (sending device), a photodetector(receiving device), and an arrayed waveguide (waveguide) are combined ona substrate. The substrate, or platform upon which the devices arecombined, is often referred to as an interposer or a sub-mount, and thecombination of the interposer with the components is often referred toas a sub-mount assembly. The sub-mount assembly is a platform forassembling and interconnecting discrete devices such as lasers,photodetectors, and other devices such as waveguides into a functionalproduct. Similar subassemblies have been used in industrial applicationsin electronics that include integrated circuits, photonic integratedcircuits, sensors, biomedical devices, among others.

Optical fibers are typically used to deliver optical signals that carryinformation to the sub-mount assemblies for processing. Optical fibersare also used to receive processed signals from the devices mounted onthe submount assembly for long and short range transmission to otherdevices, network elements, or points of use, for example. Single modefibers (SMFs), for example, can be attached to the sub-mount assembliesand aligned with optical devices that are mounted on the substrate. TheSMFs have a core diameter that is typically on the order of 10 microns.Waveguides, when implemented on the sub-mount assembly, however, can beconsiderably smaller in cross section. The significant difference insize between the mounted optical fibers that interface with thesub-mount assembly and the waveguides or optical devices that receivethe signals on the sub-mount assembly impose strict alignment andfocusing requirements on the interface between the optical fiber and thereceiving structure on the substrate. The strict alignment requirementscan necessitate costly polishing of the incoming fiber terminations.Additionally, lenses are often required between the mounted fibers andthe receiving waveguide to reduce the spot size of the incoming signalto the dimensions of the receiving waveguide. Material choices for casesin which the receiving device is a waveguide, or more particularly, aplanar waveguide, include, for example, dielectric materials such assilicon oxide, silicon nitride, and silicon oxynitride, polymers, andsemiconductors such as indium phosphide. In some instances, planarwaveguides are formed directly on a substrate as in the case of apolymer or dielectric layer, for example, and in other instances, hybridstructures are formed in which discrete waveguide devices or componentsare mounted on a substrate, interposer, or sub-mount assembly.

In addition to the use of optical fibers for delivering optical signalsto, and receiving optical signals from, the sub-mount assembly,optoelectrical and optical devices on the sub-mount assembly can also bealigned with optical devices mounted on, or in proximity to, thesub-mount assembly or interposer. In instances in which optoelectricaland optical devices are used to deliver or receive optical signals to orfrom a sub-mount assembly or interposer, the relaxed alignmenttolerances available with integrated planar waveguides provide adesirable benefit in terms of ease of manufacturing. Much of the expenseassociated with the implementation and manufacturing of photonicintegrated circuits is generally attributed to packaging. And couplingto the fiber is one of the most critical and cost intensive operations.Techniques that can be used to reduce alignment costs, or that reducethe complexity of the alignment operation are advantageous to furtheringthe state of the art and the wider acceptance in the market for packagedoptoelectronic devices. Ideally, techniques are developed that providefor passive alignment of devices and components in the optical circuits.

Thus, there is a need in the art for a waveguide structure that canreceive signals from optical fibers and optoelectrical devices withoutthe need for strict alignment tolerances or lenses.

SUMMARY

The present invention discloses a thick, planar, dual core waveguidestructure from which an optical spot size converter is fabricated. Thethick dual core waveguide structure is formed on a substrate, inembodiments, from a stack of silicon oxynitride films that exhibit lowstress and low optical signal loss. Optical spot size convertersfabricated from these thick stacks of silicon nitride films provide ameans for the coupling of optical signals to and from optical fibers andoptoelectronic devices to photonic integrated circuits (PICs).

The dual core waveguide structures enabled by the thick stackedstructure of silicon oxynitride films can provide direct and near-directoptical signal coupling to optical fibers via a lower core that issubstantially thickness-matched to the core diameter of proximallypositioned optical fibers, and a thinner upper core that facilitatessingle mode propagation for low loss signal processing.

For the thick lower core, coupling of optical signals to a PIC fromoptical fibers is achievable without unduly stringent alignmentrequirements and without the need for the lenses that are typicallyrequired to focus the incoming and outgoing optical signals in systemsfor which the optical fibers and the receiving or sending waveguides arenot thickness matched. A limitation of the thickness matched lowerwaveguide core, for thicknesses typical of single mode optical fibers onthe order of 8-10 microns, for example, is the potential for the signalto propagate in undesirable propagation modes for which signal lossescan be significant. Thick waveguide cores can be susceptible to multipleoptical propagation modes, particularly in thick planar waveguides, andmore particularly in thick planar waveguides with curvature, and as suchare susceptible to high levels of signal loss. To overcome the potentialfor incoming signal loss, and to overcome the potential for propagationin undesirable modes, incoming signals are transferred from an opticalfiber to the thick bottom core, and then to a thinner upper coredesigned for single mode propagation of the transferred optical signals.The stable, single mode propagation in waveguides such as that of theupper core is preferable for processing of the encoded optical signalsin the PICs.

In embodiments, incoming optical signals that are transferred from theoptical fibers to the thick bottom core of the dual core waveguide, aresubsequently directed from the thick bottom core, through a taperedsection, to the upper core of the dual core waveguide. The index ofrefraction of the thinner upper core is higher than that of the lowercore to facilitate the transition of the signal from the thick bottomcore to the thinner upper core effectively reducing the spot size of theincoming signals and providing a single, stable propagation mode in theupper core. In embodiments, the thickness of the upper core, in thedirection of the optical signal propagation, is gradually increased tofacilitate transfer of the signal from the lower core to the upper core.In transitioning from the thick lower core to the thinner upper corewith a higher index of refraction, the spot size of the optical signalis reduced to allow stable single mode propagation as required by thePIC.

In some embodiments, the same or a similar structure is used to expandthe spot size through a tapered section for which the thickness of thethinner upper core of the dual core waveguide is reduced along theoptical propagation path. The reduction in thickness of the upper corethrough the tapered section facilitates transfer of outgoing opticalsignals from the thin upper core into the thick bottom core, andsubsequently to optical fibers.

Using an exemplary fabrication process as described herein, the thickdual waveguide structures comprised of the thick lower core and thethinner upper core can be formed that provide low stress and low opticalsignal propagation loss, and enable the formation of the spot sizeconverter structure, also described herein. Patterning methods commonlyused in the art are used in embodiments to form precision dimensionaltolerances and to form the tapered section that facilitates thetransition of the optical signals from the thick bottom core to thethinner upper core.

The thickness of the upper single mode waveguide of the dual corewaveguide, in embodiments, affects the extent of the coupling of theoptical signals between the upper and lower waveguides. For upperwaveguide thicknesses of approximately 0.5 microns, optical signals areprimarily carried in the lower waveguide core, but can be looselycoupled to the thin upper core. An increase in the thickness of theupper waveguide core to thicknesses of approximately 1-3 microns, forexample, with the introduction of a tapered upper waveguide section,enables transitioning of the optical signals to the upper waveguidecore, and enables single mode propagation in the upper coresubstantially independent of the propagation in the lower core.

In embodiments, the upper waveguide core is increased in thicknessthrough tapered portions of the optical device circuit to provide therequired thickness in the upper core of the dual core waveguides. Theseregions of increased thickness in the upper core of the dual waveguidestructure allow for more reliable processing of the optical signals forsuch tasks as multiplexing and demultiplexing, among others,particularly in optical waveguide structures containing bends andcurvature. Curvature in the optical waveguides is necessary for, andallows for, the creation of optical device structures such as arrayedwaveguides that facilitate optical signal processing.

The tapering of the waveguides in the vertical direction, hereafterreferred to as vertical tapering, provides one approach for theformation of an adiabatic transition region for moving the signal fromthe thick bottom core to the thinner upper core of the dual corewaveguide structure. In other embodiments, lateral tapering, or taperingin the horizontal direction perpendicular to the receiving facet of thethick lower waveguide core is used in addition to the vertical taperingof the upper core. Additionally, the vertical tapering can varylinearly, nearly linearly, super-linearly, or sub-linearly with thelength of the tapered section to transition the optical signal from thelower core to the upper core of the dual core waveguide.

In embodiments, a receiving portion of the dual core waveguide isfabricated from the combination of a lower waveguide of approximately8-10 microns in thickness with a thin, upper waveguide core ofapproximately 0.5 micron in thickness. This receiving section issubstantially thickness matched to an incoming optical fiber ofapproximately 8-10 microns in thickness or to the thickness of anoptoelectronic device and receives signals from the optical fibers oroptoelectronic devices that are mounted in proximity to a receiving endor facet of the dual core waveguide structure. In these and otherembodiments, the receiving portion guides optical signals to a taperedportion through which the optical signals are substantially transitionedto a thick upper core of approximately 1-3 microns of the dual corewaveguide.

In other embodiments, outgoing optical signals propagate in thewaveguide from the thick portion of the waveguide, through the taperedportion and out the receiving portion to the optical fiber oroptoelectronic device. The same or similar device structures can be usedin embodiments to receive optical signals from optical fibers oroptoelectronic devices, and to deliver optical signals to the opticalfibers and optoelectronic devices varying only with the direction ofpropagation of the optical signals. In embodiments in which the dualcore waveguide structure is formed into a spot size converter, opticalsignals with relatively large cross-sections from optical fibers oroptoelectronic device, are reduced in cross section as these signalsprogress through the transition from the thick receiving and taperedportions to the single mode upper core of the dual core waveguide.

These and other embodiments of the dual core waveguide structure will bemade evident within the Detailed Description provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 . Cross sectional schematic drawing of a planar waveguidestructure with optical lens between a mounted optical fiber and a thinplanar waveguide (Prior Art);

FIG. 2 . (a) Cross sectional schematic drawing of a thick planarwaveguide structure that is matched in thickness to the core of amounted optical fiber, and (b) three-dimensional perspective drawing ofa thick planar waveguide structure that is matched in thickness to thecore of a mounted optical fiber;

FIG. 3 . (a) Cross sectional schematic view of a dual core waveguidestructure with a single mode waveguide core layer shown above a thickwaveguide core, and (b) three-dimensional perspective drawing of a dualcore waveguide structure with a single mode waveguide core layer shownabove a thick waveguide core;

FIG. 4 . (a) Cross sectional schematic view of a dual core waveguidestructure with a single mode waveguide layer shown above a thick planarwaveguide with a tapered adiabatic transition region, (b) crosssectional schematic view of a dual core waveguide structure showing anembodiment of the optical signal path from the bottom core to the topcore, (c) three-dimensional perspective drawing of an embodiment of adual core waveguide structure with vertical tapering, (d)three-dimensional perspective drawing of an embodiment of a dual corewaveguide structure with lateral and vertical tapering, (e)three-dimensional perspective drawing of an embodiment of a dual corewaveguide structure with lateral and vertical tapering;

FIG. 5 . (a) Top down schematic view of an exemplary optical circuitthat includes an arrayed waveguide, (b) cross-sectional schematic viewshowing upper waveguide core thickness at various locations in theoptical path from the incoming optical fiber to the output fibers, (c)top-down schematic drawing of a photonic integrated circuit showingoptical fibers aligned to dual core waveguide structures, and (d) sideview schematic drawing of a PIC with optical fibers aligned to dual corewaveguide structures;

FIG. 6 . (a) Top down view of an exemplary optical circuit that includesan arrayed waveguide showing anticipated propagation modes in the dualcore waveguide structure at various locations in the optical path fromthe incoming optical fiber to the output fibers; the anticipatedpropagation modes are in the circled insets. (b) Cross sectionalschematic view of the variation in thickness of the upper core of thedual core structure also showing the anticipated propagation modes inthe dual core waveguide structure at various locations along the opticalpath;

FIG. 7 . (a) Cross sectional schematic drawing of an exemplary dual corewaveguide structure, and (b) three-dimensional perspective drawing ofthe dual core waveguide structure shown in (a);

FIG. 8 . Cross sectional schematic drawing of an exemplary dual corewaveguide structure;

FIG. 9 . Process flow diagram for the formation of embodiments of thedual core waveguide;

FIG. 10 . Process flow diagram for the formation of embodiments of thedual core waveguide;

FIG. 11 . Process flow diagram for the formation of embodiments of thedual core waveguide;

FIG. 12 . Measured film stress in accordance with embodiments for (a)dielectric films deposited at various film thicknesses, and (b)dielectric films of various refractive indexes;

FIG. 13 . Measured optical losses in accordance with embodiments for (a)dielectric films of various refractive indexes and (b) dielectricwaveguide film structures with various bottom buffer layer filmthicknesses;

FIG. 14 . Steps for forming some embodiments of the inventive dielectricfilm structure (a) at low temperature and having low stress and lowoptical loss, (b) with each dielectric film deposited at low temperatureand having low stress and low optical loss, and (c) that include asubstrate with a buffer layer, one or more optional bottom spacerlayers, a repeating stack of one or more dielectric layers, one or moreoptional top spacer layers, and an optional top layer, followed bypattering of the stack to form a waveguide;

FIG. 15 . Schematic drawing of a typical plasma enhanced chemical vapordeposition system;

FIG. 16 . Measured refractive index for waveguide structures that weredeposited in a PECVD module over a range of gas flows for thesilicon-containing precursor;

FIG. 17 . Cross sectional schematic drawing of a system for gray scalelithographic patterning of a photoresist layer;

FIG. 18 . Embodiment using gray scale lithographic process for formingthe thinned portions and the tapered portions of the upper waveguidecore: (a) patterned mask layer, (b) after etching of the upper waveguidecore, (c) after mask removal, and (d) after subsequent lithography andetching to pattern the thin section of the upper waveguide core;

FIG. 19 . Embodiment of a method for forming the tapered portion of theupper waveguide of a dual core waveguide structure: (a) structure shownwith mask layer prior to etching of the upper waveguide layer, and (b)after etching of the upper core of the dual core waveguide layer to formthe tapered portion using an aspect ratio dependent etch process.

DETAILED DESCRIPTION

The present invention is directed to photonic integrated devices andmore particularly to dielectric waveguides and dielectric structures onsemiconductor substrates.

The ensuing description provides exemplary embodiments only, and is notintended to limit the scope, applicability, or configuration of thedisclosure. Rather, the ensuing description of the exemplary embodimentswill provide those skilled in the art with an enabling description forimplementing an exemplary embodiment. Various changes may be made in thefunction and arrangement of elements without departing from the spiritand scope as set forth in the appended claims.

A “substrate” as used herein and throughout this disclosure refers to,but is not limited to, a surface upon which planar waveguide structures,semiconductor devices, optical devices, photonic devices, optoelectronicdevices, electronic devices, and the like can be deposited, grown,placed or otherwise formed. This may include, but is not limited tosilicon, InP, GaAs, silica, a polymer, a ceramic, a metal, a glass, or acombination thereof.

An “optical waveguide”, “dielectric waveguide”, or “waveguide” as usedherein and throughout this disclosure refers to, but is not limited to,a dielectric medium or combination of medium invariant along thedirection of propagation, supporting the propagation of optical signalstypically within a predetermined wavelength range. An optical waveguidemay be at least one of an isolated structure comprising at least a coreand, in some applications, a cladding. For example, an optical fiber isa form of a waveguide, typically circular in cross section including,but not limited to flexible optical waveguides formed from extrudedglass, extruded doped silica, extruded chalcogenide glasses, andpolymer. Additionally, an optical waveguide, a dielectric waveguide, ora waveguide is a planar waveguide, formed on or within a substrate. Forexample, planar waveguides that support the propagation of opticalsignals substantially parallel to the plane of a substrate, interposer,or sub-mount assembly and includes, but is not limited to, opticalwaveguides formed within AlGaAs—GaAs material systems, InGaAsP—InPmaterial systems, ion-exchanged glass, ion-exchanged ferroelectricmaterials (e.g. proton exchanged LiNbO3), doped ferroelectric materials(e.g. titanium doped lithium niobate), silica-on-insulator,silica-on-silicon, doped silicon, ion implanted silicon, polymer onsilicon, silicon oxynitride on silicon, polymer on silicon,Silicon-On-Isolator (SOI) and polymer on polymer.

An “optical fiber” as used herein, and throughout this disclosure refersto a flexible optical waveguide that transmits optical signals over apredetermined wavelength range. This includes, but is not limited to,step-index optical fibers, graded-index optical fibers, silica opticalfibers, chalcogenide glass optical fibers, and polymer optical fibers.Such optical fibers may be multimode fibers that support multiple modes.Such optical fibers may be circular, thereby supporting multiple modesthat are at least one of laterally, vertically, and radially symmetricmodes, rectangular thereby supporting multiple modes laterally butsingle mode vertically, rectangular supporting multiple modes laterallywith limited modes vertically (e.g. 2-5), as well as waveguides withsimilar or other cross-sections. Such optical fibers may be discrete, inribbon format assembled from discrete optical fibers with discretecladdings per optical fiber, in ribbon format with common claddingbetween optical fibers, optical fibers embedded in a polymer flexiblefilm, and optical fibers attached to a polymer flexible film.

A “waveguide core” as used herein, and throughout this disclosure refersto the signal carrying portion of a waveguide through which asubstantial portion of an optical signal propagates. A “dual corewaveguide” is a waveguide or waveguide structure, typically a planarwaveguide, comprised of two waveguide cores. A “multicore waveguide” isa waveguide or waveguide structure, typically a planar waveguide,comprised of two or more waveguide cores

“Silicon oxynitride” as used herein, and throughout this disclosure,refers to materials comprised of stoichiometric and non-stoichiometriccombinations of the elements of silicon, oxygen, and nitrogen, andincludes silicon oxides and silicon nitrides. “Silicon oxynitride” filmsmay be doped, either intentionally or unintentionally, and may containdesirable and undesirable impurities. Examples of materials that mightbe intentionally or unintentionally incorporated into silicon oxynitrideinclude hydrogen, phosphorous, boron, sodium, among others. Therefractive indices of “Silicon oxynitride” films typically lie withinthe range of approximately 1.4 to 2.0.

A “multiplexer” (MUX) as used herein, and throughout this disclosure,refers to a device that combines a plurality of source channels andprovides a single combined output. This includes, but is not limited to,passive multiplexers, active multiplexers with transmitters andwavelength division multiplexers, active multiplexers with receivers,transmitters and wavelength division multiplexers, unidirectionalmultiplexers and bidirectional multiplexers.

A “demultiplexer” (DMUX) as used herein, and throughout this disclosure,refers to a device that receives multiple signals from a single inputline or channel and routes these signals into multiple output lines orchannels. This includes, but is not limited to, passive demultiplexers,active demultiplexers with receivers and wavelength divisionmultiplexers, active demultiplexers with receivers, transmitters andwavelength division multiplexers, and unidirectional demultiplexers.

An “interposer” as used herein and throughout this disclosure refers to,but is not limited to, a substrate that provides mechanical support andelectrical or optical interface routing from one or more electrical,optical, and optoelectrical devices to another. Interposers aretypically used to route optical or electrical connections from variousdevices or die that are mounted on, or connected to, the interposer. An“optical interposer” is an interposer that provides for the opticalinterfacing between optical devices mounted or connected thereon.

A “sub-mount assembly” as used herein and throughout this disclosurerefers to, but is not limited to, an assembly that includes a substrate,typically an interposer, that is populated with one or more optical,optoelectrical, and electrical devices.

A “sub-mount or submount” as used herein and throughout this disclosurerefers to, but is not limited to, a substrate used in a sub-mountassembly, such as a substrate, an interposer, or any type of mechanicalsupport structure.

A “substrate” as used herein and throughout this disclosure refers to,but is not limited to, a mechanical support upon which an interposer isformed. Substrates may include, but not be limited to, silicon, indiumphosphide, gallium arsenide, silicon, silicon oxide-on-silicon, silicondioxide-on-silicon, silica-on-polymer, glass, a metal, a ceramic, apolymer, or a combination thereof. Substrates may include asemiconductor or other substrate material, and one or more layers ofmaterials such as those used in the formation of an interconnect layer.

“Propagation mode” as used herein and throughout this disclosure refersto, but is not limited to, the characteristic light intensitydistribution or field intensity distribution of propagating light, infor example free space or within a waveguide.

The “effective index of refraction”, as used herein and throughout thisdisclosure, refers to the value of the mode effective index ofrefraction for a composite waveguide consisting of multiple layers.

Referring to FIG. 1 a cross sectional schematic view of a planarwaveguide structure on a substrate is shown. In a planar waveguide,optical signals travel in a substantially horizontal direction parallelor substantially parallel to the plane of the substrate surface as shownin the prior art depicted in FIG. 1 . One or more optical fibers 180 aretypically mounted to the periphery of a substrate 110, interposer, orsub-mount assembly to deliver optical signals 102 to the planarwaveguide 112. In many applications, the optical fiber 180 is a singlemode fiber of circular cross section with a core diameter ofapproximately 8-10 microns. In silicon photonic systems, the thicknessof planar waveguides 112 is typically on the order of a few microns, andis limited by the buildup of stress. And although not required, thewidth of the planar waveguide is typically of the same dimension as thethickness, thus creating a square cross-section for the planarwaveguide.

In the prior art shown in FIG. 1 , a lens 50 is inserted in the opticalpath between the optical fiber 180 mounted at the edge of the substrate110 and the planar waveguide 112 to focus the optical signal 102 fromthe relatively large core 182 of the optical fiber 180 to the muchthinner planar waveguide 112. The diameter of a typical single modeoptical fiber 180 used in communication networks for the transmission ofoptical wavelengths in the range of 1100 to 1600 nm is typically 8-10microns. Typical dielectric planar waveguides, on the other hand, are onthe order of less than a micron to a few microns in thickness. Ingeneral, dielectric waveguides are susceptible to increasing stress withincreasing film thickness, hence limiting the tolerable thicknesses ofdielectric waveguides. Dimensional differences between the diameter ofthe core 182 of the optical fiber 180 and the planar waveguides 112 inmany applications that utilize dielectric waveguides, require tightalignment tolerances between the planar waveguides 112 and the opticalfibers 180 to limit losses in the optical signal 102 in transitioningfrom the optical fiber 180 to the planar waveguide 112. The tightspatial and angular alignment tolerances and the need for lens 50between the end 184 of the optical fiber 180 and the end facet 185 ofthe planar waveguide 112, increase the complexity of the overallassembly relative to preferable alternatives in which the tolerances canbe widened and for which the lenses 50 can be eliminated. Tightalignment tolerances can also require costly polishing or finishing ofthe fiber termination 184. Alternatively, passive alignment techniques,if available, are preferable over the active alignment techniques thatrequire a more complex procedure for providing suitable alignment andoptimization of the signal transmission across the optical fiber/planarwaveguide interface.

Referring to FIG. 2 a , an optical fiber 280 with cladding 281 is shownin proximity to the edge facet 285 of a thick planar waveguide 220 onsubstrate 210. The edge facet 284 is substantially aligned to the planarwaveguide edge facet 285 without the requirement for the lens 50 (as wasshown in FIG. 1 .) A three-dimensional perspective drawing of thisstructure is shown in FIG. 2 b . Thick planar waveguides enable directtransmission of the optical signal 202 from an optical fiber 280, or asimilarly positioned optoelectronic or optical device without therequirement for the focusing lens 50 that is shown in FIG. 1 . Thickplanar waveguides on the order of the diameter of the core 282 of theoptical fiber 280 are known in the art, and are typically polymers.Dielectric films are preferred over polymers due to their inherentdimensional and material stability, the capability to control theoptical properties of these materials, and the available knowledge basefor the formation and patterning of these films, among other benefits.Thick dielectric film structures, however, are susceptible toprohibitively increasing stress with increasing film thickness. Highfilm stress can lead to deformation of the substrate, delamination ofthe films, and other undesirable effects.

In addition to the potential problems that arise with the formation ofthick dielectric planar waveguides, planar waveguides with thicknesseson the order of the core diameter of a typical single mode optical fibercan allow for optical signal propagation in undesirable or non-opticalmodes. Preferably, optical signals are limited to single modepropagation once the signals have been received into a photonicintegrated circuit, for example, for signal processing operations suchas multiplexing and demultiplexing, among others.

In Provisional application 62/621,659, a thick dielectric structuresuitable for use in forming thick planar waveguides, on the order of thediameter of single mode fibers, is included for reference herein in itsentirety. In Provisional application 62/621,659, thick dielectric filmstructures are formed from stacks of silicon oxide and siliconoxynitride layers with low stress and with controllable opticalproperties that are suitable in some embodiments for receiving opticalsignals from proximally positioned optical fibers without stringentalignment requirements and without the requirement for the use of lenses50 as further described herein. The advantages of thick siliconoxynitride film structures for use in planar waveguides includedimensional stability, controllable optical properties, the availabilityof known patterning methods, and the capability to achieve passivealignment of optical fibers and planar waveguides, among others.

Referring to FIG. 3 a , a dual core waveguide 360, in embodiments, isshown comprised of a thick lower core 342 and a thinner upper core 354.The dual core planar waveguide structure 360, in some embodiments,utilizes a thick dielectric film structure 342, similar to the thickfilm structure 220 described in FIG. 2 , but with the addition of asecond waveguide core 354 positioned above the thick lower core 342. Athree-dimensional perspective drawing of this structure is shown in FIG.3 b . In some embodiments, the thick lower waveguide core 342 issubstantially thickness-matched to the core diameter of one or moremounted single mode optical fibers 380. Generally, as the thickness of aplanar waveguide is increased, the susceptibility for optical signalstraveling within the waveguide to propagate in less desirable,non-fundamental propagation modes is increased. The magnitude of thethickness of the lower waveguide core 342, renders optical signalstraveling within it more susceptible to transitioning to higher orderpropagation modes.

The addition of the single mode waveguide 354 over the thicker multimodewaveguide 342 provides a parallel optical signal propagation path withinwhich preferable single mode propagation can be enabled and directed asdescribed herein. In embodiments, for upper waveguide core 354 on theorder of 0.5 microns in thickness, optical signal 302 traveling in thedual core structure remains weakly coupled between the two cores. As thethickness of the upper core 342 is increased, and with proper selectionof the relative indices of refraction of the lower core 342 and theupper core 354, the propagation of the optical signals can beeffectively directed between the two cores as further described herein.Structures, such as those shown in FIG. 3 , provide firstly a thickwaveguide structure 342 for receiving an optical signal 302 from anoptical fiber 380 with core 380 and cladding layer 381 or optoelectronicdevice (not shown), without the requirements for tight dimensionaltolerances of the edge facets 384, 385, or without the requirements fora focusing lens 50, or both, and secondly, a thin waveguide structure354 for single mode propagation of optical signals 302. Tightdimensional tolerances include the surface finish of the facet surfacesfor the optical fiber and the planar waveguide, and the distance andparallelism between the end of the optical fiber and the edge of theplanar waveguide adjacent to the optical fiber, for example.

In some embodiments, the single mode core 354 of a dual core waveguideis positioned below the thicker core 342.

Referring to FIG. 4 a , a cross sectional illustration of the dual corewaveguide 460 is shown in which the upper waveguide core 454 iscomprised of a thin portion 455, a tapered portion 457, and a thickportion. The index of refraction is higher for the upper waveguide core454 relative to the thick bottom core 442. An incoming optical signal402 propagates from the core 482 of the optical fiber (with claddinglayer 481) to the thick bottom core 442, and in embodiments, a portionof the optical signal 402 may propagate in the thin portion 455 of theupper waveguide core 454. Some coupling of the optical signal 402 isanticipated in some embodiments between the thin waveguide core 455 andthe thick bottom core 442 as the signal propagates substantially throughthe portion of the thick bottom core 442 that lies beneath the thinupper waveguide core 455. Optical signal 402 propagates through thethick portion 442 beneath the thin upper waveguide core 455 to thetapered portion 457 of the upper waveguide core 454. The optical signal402 adiabatically transitions from a region of lower index of refractionin the lower core 442 to the region of higher index of refraction in theupper core 454. That is to say, that in transitioning, the propagationof the optical signal moves substantially to the upper core 454. Intransitioning to the single mode upper core 454 of the dual corewaveguide structure 460, the optical signal 402 becomes substantiallynon-coupled between the two waveguides, propagating primarily in thethick portion of the upper core 454, and less so in the bottom core 442.

Referring to FIG. 4 b , the anticipated behavior of an optical signal402 traveling from the optical fiber 480, through the weakly coupledthin portion 455 of the upper waveguide core 454, and then through thetapered portion 457 and into the thick portion of the upper waveguidecore 454 is shown. The incoming signal 402 from the optical fiber 480,which is circular in cross section, typically propagates in a singlemode. As the optical signal, however, moves into the thick planarwaveguide 442, in some embodiments, it becomes susceptible topropagating in a number of higher order modes. The susceptibility totransition to higher order propagation modes is largely due to thethickness and shape of the lower waveguide core, the thickness of whichcan be on the order of 6-15 microns. In embodiments in which the opticalsignal 402 exhibits propagation in a higher order mode, the propagationis not expected to be significantly affected by the presence of the thinweakly coupled upper waveguide portion 455, although the optical signal402 has a propensity to move into the thin portion 455 in typicalcircumstances in which the upper waveguide core 454 has a higher indexof refraction relative to the lower core 442. As the optical signal 402transitions adiabatically in the tapered portion 457 of the upperwaveguide core 454, propagation of the optical signal 402 transitions toa more stable single mode form in the upper core 454 than in the lessrestrictive lower core 442. Single mode propagation is preferable inmany optical devices used in optical signal processing. Conversion ofthe single mode signal in the lower core 442 to the more confined andmore stably directed single mode signal in the upper core 454 can beaccomplished with a spot size converter, the function for which isprovided with the dual waveguide structure shown in FIG. 4 . That is,the dual core structure shown in FIGS. 4 a and 4 b , in embodiments,performs the function of a spot size converter. The optical signal 402from the single mode fiber 480 that enters the lower core 442 of thedual core waveguide 460 is essentially a large diameter single modesignal that is redirected and confined into a more stable form with asmaller cross section or spot size within the thinner upper core 454.While the increased thickness of the bottom core 442 can allow forpropagation in a number of higher order modes in some embodiments, thisincreased thickness is more suited to the reception of the opticalsignal 402 from the substantial thickness matching between the lowercore 442 and the optical fiber core 482 in comparison to an approach ofattempting to guide the optical signal 402 directly into the thinnerupper core 442, or any form of thin planar waveguide in which theoptical fiber core 482 is not substantially thickness matched. Thethickness matching between the optical fiber core 482 and the thicklower core 442 that receives the optical signal 402 is beneficial for anumber of reasons that include the ability to eliminate the use oflenses between the end of the optical fiber 484 and the planar waveguideend facet 485, the ability to significantly reduce, relax, or eliminatethe polishing of the optical fiber end 484, and the ability to toleratereduced alignment tolerances between the optical fiber end 484 and theplanar waveguide end 485 in the fiber-to-substrate mating configuration.

The portion of the dual waveguide structure 460 within which the uppercore 454 is a thinned upper core region 455, on the order of 0.5 micronsin thickness for a silicon oxynitride film, for example, typically hasan index of refraction that is higher than that of the lower core 442 topromote movement of the optical signal 402 from the lower core 442 intothe upper core 454. The thickness of the thick portion of the upper core454 for comparison is approximately 2 microns for a silicon oxynitridefilm, a thickness that provides a preferentially more stable single modepropagation for the optical signal 402 relative to signals propagatingin the much thicker lower core 442. The thicknesses of the upper core454 and the lower core 442 can vary, as can the stoichiometry of thesilicon oxynitride used in these layers.

The tapered transition region 457 between the lower core 442 and theupper core 454 is provided in numerous ways, in embodiments, toeffectively reduce the volume through which the optical signal 402propagates as this signal transitions from the high-volume lower core442 to the reduced volume of the upper core 454. In general, tapering isaccomplished, in embodiments, with vertical tapering or a combination ofvertical and lateral tapering.

In a vertically tapered upper core waveguide section 457, the thicknessof the upper core 454 in tapered transition region 457 varies as shown,for example, in FIG. 4 c . FIG. 4 c shows a three-dimensionalillustration of the upper waveguide core 454 in which a thin upperwaveguide core section 455 is increased in thickness through the taperedsection 457 to a thicker layer region 454 beyond the tapered section457. In embodiments in which the tapered section 457 is only verticallytapered or substantially vertically tapered, the change in thickness ofthe upper core 454 is the only change, or at least the primary change inthe dimension of the cross-section of the upper waveguide core 454. Inupper core waveguides that are vertically tapered, a largecross-sectional optical signal 402 propagating in the lower core isdirected upward in embodiments from an increase in the refractive indexin the upper core 454 through the vertically tapered section 457, and asthe signal is guided through the tapered section 457, the crosssectional area of the signal 402 is reduced and is ultimately confinedto the cross sectional area of the thick portion of the upper waveguidecore 454. The volume reduction of the upper core of the dual corewaveguide 460 effectively acts to reduce the spot size or crosssectional area of the optical signal 402 through the transition portion457 and the directed movement of the optical signal 402 with theincreased index of refraction in the upper core, which promotes movementof the signal 402 from the larger lower core 442 to the smaller uppercore 454.

Alternatively, the tapered section 457 can consist of vertical andlateral tapering in which both the thickness and the width of the uppercore 454 change with position from the thin upper core portion 455through the tapered section 457 to the thick portion of the upperwaveguide core 454 as illustrated in FIG. 4 d and FIG. 4 e . In theembodiment shown in FIG. 4 d , the thin upper waveguide core portion 455is shown to narrow in width along a direction parallel to the opticalsignal propagation path. Also shown in the embodiment in FIG. 4 d , thewidth of the upper waveguide core 454 continues to narrow in the taperedsection 457. The height of the upper waveguide core 454 increases in thetapered portion 457 in the embodiment shown in FIG. 4 d , as was thecase in the embodiment shown in FIG. 4 c , until the waveguide core 454reaches its full thickness beyond the tapered section 457. Conversely,in the embodiment shown in FIG. 4 e , the thin upper waveguide portion455 is shown to increase in width in the direction parallel to theoptical signal propagation path. The width of the upper waveguide core454 continues to widen in the tapered portion 457 in the embodimentshown in FIG. 4 e . The height of the upper waveguide core 454 increasedin the tapered portion 457 in the embodiment shown in FIG. 4 e until thewaveguide core 454 reaches its full thickness beyond the tapered section457.

The vertical tapering in embodiments as shown in FIGS. 4 c and 4 d , forexample, can consist of a wide range of continuous or discontinuouschanges in the vertical dimension of the upper core 454. Similarly,lateral tapering of the upper core 454 of the dual core waveguide 460can also consist of any continuous or discontinuous change in thelateral dimension of the upper core 454 as the upper waveguidetransitions from the thin portion 455 through the tapered section 457 tothe thick portion 454 as shown in FIG. 4 d.

As has been noted, the dual core waveguide 460 is comprised of the lowerwaveguide core 442 and the upper waveguide core 454, and upper waveguidecore 454 is comprised of a thin portion 455, a tapered portion 457, anda thick portion. In a dual core waveguide structure with both lateraland vertical tapering, the width of the thin upper waveguide coresection 455 transitions to a different width in at least a portion ofthe tapered section 457 to at least an initial width for the thick upperwaveguide region 454 as shown in FIG. 4 d . In some embodiments, thewidth of the upper core in the tapered section 457 is variedcontinuously from the start of the tapered section 457 to the end of thetapered section 457. In other embodiments, the width of the upper corein the tapered section 457 is varied in multiple stages, with partialdecreases in the width of the upper waveguide core 454 with each stage.

The lateral transition in the width of the thin section 455 of the upperwaveguide core 454 in an embodiment is a linear transition, with alinearly changing reduction in the width of the waveguide 455. In otherembodiments, the changes in the width of the waveguide section 455changes super-linearly with distance from the edge facet 485. In yetother embodiments, all or a portion of the waveguide section 455 changeswith a sub-linear dependence with distance from the edge facet 485. Insome embodiments, the thickness of the thin upper waveguide core section455 can be varied with distance along the direction of propagation.

In other embodiments, the width of the upper core in the tapered section457 is varied continuously and in steps as the width of the upper core454 of the dual core waveguide 460 transitions from the thin upper coresection 455 to the thick upper core section 454. Stepped transitions inlateral tapering, and vertical tapering as well, may be preferable insome embodiments, to simplify the lithographic patterning and subsequentetch processes. A stepped vertical transition might be implemented witha number of photomasks and etch steps, for example, to accomplish thetransition from the thin portion 455 to the thick portion of the upperwaveguide core 454.

As the signal 402 transitions through the portion of the dual waveguidestructure 460 in which the upper waveguide core is a tapered section 457and into the thick portion of the upper waveguide 454, the opticalsignal 402 transitions to having a preferable and stable single modepropagation characteristic. Maintaining a single fundamental propagationmode is preferable in many subsequent stages in the optical circuitwithin which the optical signal is processed, decoded, or encoded. Thelarge diameter signal 402 a (as shown in FIGS. 4 c and 4 d ) receivedfrom the optical fibers 480 transitions to a smaller diameter signal inthe spot size conversion structures in FIG. 4 c and FIG. 4 d as shown.The cross-sectional diameter of the propagating optical signal, a largediameter signal 402 a in the lower waveguide core 442 that lies belowthe thin upper core segment 455, is reduced in cross-section as forexample, optical signal 402 b, as it transitions through tapered section457 to the smaller cross sectional signal 402 c in the upper core 454.

Referring to FIG. 5 a , a top-down view is shown of an exemplary opticalcircuit element 524 on a sub-mount 520 in which some beneficial featuresof the dual core waveguide structure 560 are described. In anembodiment, an optical device 524 is formed from a dual core waveguidestructure 560 on a substrate, interposer, or sub-mount 520 with opticalfibers 580. Optical fibers 580 can be used to deliver optical signalsto, or receive optical signals from, waveguides and devices on sub-mount520. For simplicity, the structure 520 is hereafter referred to as asub-mount. Sub-mount 520 can be a substrate, an interposer, a sub-mount,a sub-mount assembly, or any type of mechanical support structure forwhich the dual core waveguide can be formed, or mounted. Optical circuitelement 524 in the embodiment shown in FIG. 5 is an arrayed waveguide,although the optical element 524 can be any optical device, waveguide,optoelectrical device, or electro-optical device to which the inventivedual core waveguide 560 on sub-mount 520 is used to transfer, receive,deliver, or propagate optical signals 502 from either optical fibers 580or an optical or optoelectronic device. A dual core planar waveguidestructure 560 is formed on the submount 520 in embodiments in filmsformed on the sub-mount 520. In other embodiments, the dual core planarwaveguide is a discrete device that is formed partly or in its entiretyand then mounted on the sub-mount 520. In an embodiment, optical fiber580 provides optical signals 502 to lower core 542 of the dual corewaveguide 560 at an input location on the sub-mount 520. V-grooves (asshown in FIGS. 2 and 3 but not highlighted in FIG. 5 ) in the substrateor sub-mount 520 are used in some embodiments to facilitate the mountingof optical fibers 580, 582.

Other details of the submount 520 shown in FIG. 5 a include the array ofthe upper waveguide cores 554 and the spacer layer 550. In theembodiment shown in FIG. 5 , the field area shows spacer layer 550. Inother embodiments, the spacer layer 550 and the bottom waveguide core542 are also patterned. Patterning of the bottom core 542, in someembodiments, can improve the performance of the arrayed waveguide. Thewidth of the bottom core 542 in some embodiments is wider than thethickness of the bottom core 542, but not so wide as to produceundesirable interference between the optical signals in adjacentwaveguides.

FIGS. 5 a and 5 b show schematic views of an embodiment of a dual corewaveguide configured with an optical device, namely an arrayedwaveguide. The structure of the dual core waveguide provides favorablebenefits to the coupling of arrayed waveguides and optical fibers asdescribed herein. The selection of the arrayed waveguide as an exemplarystructure is not intended to limit the applicability of the dual corewaveguide to this or other optical and optoelectrical devices, butrather is shown as an exemplary embodiment of a type of device for whichthe dual core waveguide structure and the spot size converter formedtherefrom, can be favorably implemented.

In an exemplary optical device configuration as shown in thecross-section view in FIG. 5 b , the optical signal 502 enters the lowercore 542 (at left side of FIG. 5 ) of the dual core waveguide 560 Thethickness of the lower core 542 of the dual core waveguide 560 issubstantially matched to the core of the optical fiber 580, as shown.For reference, the core of the optical fiber is the portion of the fiberthrough which the optical signal substantially propagates, in contrastto the cladding or sheath layers, which are typically much thicker thanthe core diameter. Optical signal 502 is received by the lower core 542of the dual core waveguide 560, and is weakly coupled to the upper core555 a as it propagates in the lower waveguide core 542. In general, thedifferences in the refractive index, typically lower in the bottom core542 and higher in the upper core 554, will promote movement of theoptical signal 502 to the region of higher refractive index in the uppercore. In the thin upper core section 555 a, the thinness of the uppercore and the correspondingly weak coupling prevent movement andpropagation into the upper core, but as the signal further propagates tothe tapered section 557 a, the optical signal will undergo an adiabatictransition from the lower core 542 through the tapered section 557 a(not to scale) to the thick upper core of waveguide core 554 a of thedual core waveguide 560, and in doing so, will undergo a reduction inthe cross-sectional size of the propagating signal 502 as it moves intothe upper core 554 a due to the smaller volume of the upper waveguidecore 554 in comparison to the lower core 542. For the purposes ofclarification, the suffix “a” denotes the input side of the structureshown in FIG. 5 , and is intended to distinguish from the output side(denoted with a “b”) of the structure as shown at the right side of thestructures shown in FIG. 5 . In the absence of an “a” or “b”designation, either designation is implied. In some embodiments, theoptical signals enter the structure shown at the left side of thecross-sectional structure shown in FIG. 5 b and first encounter thethinned portion 555 a of the upper waveguide core and subsequently thetapered section 557 a, and the thick upper core portion of the waveguidecore 554 a. Upon propagation of the signal through the optical device524, the optical signal exits at the right side of the cross-sectionalstructure shown in FIG. 5 b encountering the output sections of theupper waveguide cores 554 b, the tapered section 557 b, and the thinnedportion 555 b. As shown in the top-down view, multiple upper cores 524 bare formed at the output of the arrayed waveguide 524. In thisembodiment, the combined sub-mount 520 with arrayed waveguide 524 is ademultiplexer device. Alternatively, the input signals are provided inthe optical fibers 582 at the right side of the device structure shownin the cross section of FIG. 5 b , directed at first into the thin uppercore section 555 b, tapered section 557 b, and thick waveguide core 554b, and then through the arrayed waveguide 524, and into the thickwaveguide core 554 a, tapered section 557 a, and thinned section 555 a,and output from the substrate through optical fiber 580. In thisconfiguration, the device functions as a multiplexer.

The configurations described above are intended to illustrate keyelements in practice for the dual core waveguide structure 560 in whichthe dual core waveguide 560 is formed onto a substrate 520, and furtherformed into a spot size converter, and in which the spot size converteris incorporated into optical devices such as a multiplexer ordemultiplexer. In practice, the optical device 524 can be any opticaldevice that benefits from the features of a thick optical signalreceiving waveguide and a thinner single mode waveguide. In embodiments,the optical device 524 can be one or more optical devices. In otherembodiments, the optical device 524 is one or more of a waveguide, agrating, a filter, a blocker, a prism, a combiner, a multiplexer, ade-multiplexer, a splitter, or any of a wide range and type, orcombinations of optical devices. The thick receiving waveguide 542 issubstantially thickness-matched to the cores of mounted optical fibersand the upper waveguide core 554 is a single mode core that allows forsubstantially decoupled propagation of an optical signal that issignificantly smaller in cross section than the incoming optical signal502. In embodiments, a tapered section 557 is formed to facilitate thetransition of the optical signal from the receiving core 542 to thesingle mode core 554.

In some embodiments, the combined sections 555, 557, and 554 provide thefunctionality of a spot size converter to provide single mode opticalsignal propagation in the upper core 554 of the dual core waveguide 560.Single mode optical signal 502 propagates with the reducedcross-sectional size of the upper waveguide core 554 relative to thelower core 542 in exemplary embodiments. In some embodiments, thefurther propagation of the optical signal 502 beyond the tapered section557 a in the optical device 524 on sub-mount 520 occurs substantially inthe upper core 554 of the dual core waveguide 560.

Upper waveguide core 554 can include a waveguide element, such as amultiplexer or other optical device, to split the signal.

The arrayed waveguide 524, in an embodiment, is a demultiplexing devicethat provides a means for the separation of a composite incoming signal,consisting of multiple wavelengths of light, into its constituentsignals. The arrayed waveguide 524 is an example of an optical devicethat benefits from the features of the inventive dual core waveguide560, and particularly in embodiments in which a spot size converter isformed from the dual core waveguide 560. In an embodiment, a spot sizeconverter is formed from the dual core waveguide 560 to receive anoptical signal 502 in lower waveguide core 524 and convert a largecross-sectional optical signal 502 to an optical signal with asubstantially smaller cross-section through a tapered section 557 formedin the upper core 554 of the dual waveguide structure 560. Other opticaldevice structures and elements can also benefit from the dual corewaveguide structure 560. Progression of the optical signal through thedemultiplexing arrayed waveguide 524 results in a set of optical signalsthat are delivered to output optical fibers 582 in the embodiment shown.For simplicity, only three optical fibers are shown although one or anynumber of optical fibers 582 might be positioned at the output of anarrayed waveguide 524, for which the arrayed waveguide 524 is configuredas a demultiplexer, to deliver the optical signals to a location on thesame sub-mount 520 or elsewhere through output fibers 582, for example,for further processing. Conversely, in instances in which the arrayedwaveguide 524 is a multiplexer, and optical fibers 582 provide theinputs to the arrayed waveguide, one or any number of optical fibers 582can be positioned at the input of the arrayed waveguide 524, for whichthe arrayed waveguide is configured as a multiplexer, to deliver theoptical signals to the arrayed waveguide 524.

In an embodiment of the exemplary structure shown in FIGS. 5 a and 5 b ,fiber 580 delivers an optical signal to the thick lower waveguide 542,through tapered section 557 a (not to scale) to the arrayed waveguidestructure 524, and then again through tapered sections 557 b at theoutput of an arrayed waveguide 524 that is configured for demultiplexingthe incoming optical signal 502. For clarity in this exemplarydescription, the optical fibers 582 are on the output side of thearrayed waveguide 524. In embodiments, the arrayed waveguide 524 is ademultiplexing device from which the output of the arrayed waveguide 524is directed into output fibers 582.

In yet other embodiments, the optical fibers 582 deliver one or moreoptical signals 502 to the arrayed waveguide 524 for multiplexing of thesignals, and the output of the arrayed waveguide is directed to theoptical fiber 580. In these embodiments, multiple optical signals arecombined to form an output signal 502 that is delivered to optical fiber580 from the thick waveguide core 542. In embodiments in which theoptical fibers 582 or multiple planar waveguides deliver signals to thearrayed waveguide 524, the optical signals propagate from the opticalfibers 582, into the thick bottom cores 542 b, are directed upward tothe tapered section 557 b, and into the upper waveguide core 554 b. Uponrecombination in the arrayed waveguide 524, a single optical signal 502that is a composite signal with multiple constituent wavelengths,propagates through the thick portion 554 a of the upper waveguide coreto the tapered section 557 a within which the optical signal 502 is freeto adiabatically expand into the thick lower core section 542 in theportion of the dual core waveguide 560 with the thin upper core 555 a.As the optical signal 502 is expanded in the lower core 542, it canagain be coupled to the optical fiber 580.

In other embodiments, the optical signals 502 are guided into additionaloptical devices, optoelectrical devices, waveguides, for example, afterthe demultiplexing in the arrayed waveguide 524. In FIGS. 5 a and 5 b ,the optical device 524 is an arrayed waveguide. In other embodiments,the optical device 524 is an echelle grating, or other optical oroptoelectrical device. Referring to FIGS. 5 c and 5 d , schematic viewsare shown in which the device 524 a on submount 520 is one or more of anechelle grating or other optical or optoelectrical devices configured asa component of a PIC. One or more optical fibers 580 are mounted orotherwise configured in proximity to the substrate, interposer, orsub-mount 510, such that the optical signals can be transferred from theone or more optical fibers to one or more components of the PIC. In theconfiguration shown in FIGS. 5 c and d , the one or more optical fibers580 are aligned with dual core waveguide structures comprised of a thicklower core 542 and upper core 554. The thickness of the upper core 554can vary with distance from the optical fiber 580 as shown in theexemplary embodiment in the schematic cross section provided in FIG. 5 d. The cross-sectional schematic shows the thick lower core 542 insubstantial alignment to the core of the optical fiber 580. The core ofeach of the optical fibers 580 is represented by the dotted lines withinthe top down and side views of the optical fiber 580. The thickness ofthe lower core 542 of the dual core waveguide is substantially matchedto the diameter of the core of the optical fiber 580. In the figure, thecladding of the optical fiber is not shown to scale, and is typicallymuch thicker than the core diameter. Accommodations for the cladding aretypically made in the size of the V-groove within which the opticalfibers 580 are supported, such that the core of the optical fiber 580,582 are aligned to the receiving core 542 of the dual core waveguide.Additionally, the thickness of the substrate 510, also not drawn toscale, is typically much greater than shown in FIG. 5 relative to thethickness of the core of the optical fiber 580 than what is depicted inthe drawing.

In an exemplary embodiment of the dual core waveguide configured with aPIC as depicted in FIG. 5 c , for example, optical signals can beinbound from one or more of the optical fibers 580 to the PIC, oroutbound from the PIC to one or more of the optical fibers. PICs can becomprised of one or more optical or optoelectrical device, and the dualcore waveguides can be used in embodiments to deliver optical signalsto, and receive optical signals from, the optical fibers 580 and thePIC. Additionally, the dual core waveguide structures can be used toprovide optical connections between the various optical andoptoelectrical devices 524 a, 524 b, 524 c, 524 d within the PIC asshown in FIG. 5 c . Optical device 524 a Metallization lines 588 arealso shown in FIG. 5 c and FIG. 5 d that provide electricallyinterconnections for electrical and optoelectrical devices within thePIC. Electrical contacts 589 for connections to externally mounteddevices, for examples, are also shown in FIGS. 5 c and 5 d.

Referring to FIG. 6 , the planar optical circuit of FIG. 5 consisting ofan arrayed waveguide device 524, is shown with anticipated propagationprofiles of the optical signal 502 at various locations in the opticalcircuit. In the exemplary optical circuit shown in FIG. 6 , an incomingoptical signal is provided to the sub-mount 520 in the attached opticalfiber 580 at the left-bottom of FIG. 6 a , and propagates from the leftedge of the device, through the arrayed waveguide 524, and out theright-bottom part of the circuit to the output fibers 582. The input toan arrayed waveguide, in some embodiments, is a multiplexed opticalsignal consisting of a number of distinct wavelengths of light. In anexemplary embodiment, the incoming optical signal is a composite signalof 16 wavelengths, centered around a wavelength of 1550 nm withincrements of 20 nm between each sub-signal of the combined incomingsignal. Other wavelengths and spacings between wavelengths are used inother embodiments. Typical wavelengths of light that are commonly usedin optical communication networks are in the range of 1200 to 1700 nm. Acommonly used wavelength, for example, is 1550 nm. In an embodiment, theincoming optical signal 502 is a multiplexed signal, and consists, forexample, of a set of 16 different wavelengths, centered around 1550 nmin increments of 20 nm. In another embodiment, the incoming opticalsignal is a multiplexed signal consisting of eight wavelengths of light,centered around 1300 nm, in increments of 20 nm. In yet anotherembodiment, the incoming optical signal 502 is a multiplexed signalconsisting of four wavelengths centered around 850 nm in increments of15 nm. An arrayed waveguide provides a means for separating the variouswavelengths in the incoming optical signal, and then providing distinctphysical channels within which to direct the individual signals. In thisexample, the circuit contains sixteen output fibers 582, for example, toprovide a unique channel for each of the demultiplexed wavelengths fromthe incoming signal 502.

Again referring to FIG. 6 , the anticipated propagation modes are shownin the circled insets 571, 573, 575. (The suffix “a” attached to theinsets refers to the input side of the device to the left of FIG. 6 andthe suffix “b” refers to the output side to the right of FIG. 6 .) Theanticipated propagation mode, for example, for a typical incomingoptical signal 502 in optical fiber 580 is axially symmetric as shown ininset 571 a for the incoming optical fiber 580. As the optical signal502 enters the thick lower core 542 of the dual core waveguide 560, ahigher refractive index in the upper core 554 promotes some movement ofthe optical signal 502 to the upper core 555 a, creating the asymmetricsignal characteristic in the thin portion 555 a of the dual corewaveguide 560 as shown in the inset 573 a. Full movement of the opticalsignal is limited by the physical volume in the upper core in the thinupper core segment 555 a.

In embodiments, the length of the region 555 a in the direction ofpropagation of the optical signal 502 will affect the intensity of theoptical signal 502 at the input to the tapered section 557 a. Inembodiments, the section 555 a is a straight section without curvature.In other embodiments, the section 555 a is straight section withoutcurvature and with a length in the direction of propagation of theoptical signal 502 such that the signal intensity at the input to thetapered section is a maximum or near maximum. In other embodiments, thesection 555 a is straight section without curvature and with a length inthe direction of propagation of the optical signal 502 such that thesignal intensity at the input to the tapered section is at leastadequate for signal processing. In an embodiment, the length of thesection 555 a is in the range of 100 to 500 microns. In otherembodiments, the length of the section 555 a is in the range of 50 to1000 microns. In yet other embodiments, the length of the section 555 ais such that a discernable signal is detected beyond the tapered section557 a for subsequent optical signal processing.

Further propagation of the optical signal 502 along the dual corewaveguide structure 560 through the tapered section 557 a has theanticipated result as shown in the inset 575 a in which the signal issubstantially present in the single mode upper core 554 of the dual corewaveguide 560. In this section of the upper core 554, the optical signalis not limited by the waveguide volume as in segment 555 a, but ratheris confined to the smaller volume and higher index of refraction of theupper core 554, but with an increased susceptibility to propagate insingle mode form. The approximate locations of the outline of thewaveguides, relative to the modeled fields, for each of the insets isshown as a white outline. In inset 571 a, for example, the white outlinedepicts the circular cross section of the core of the optical fiber.Similar outlines, rectangular in cross section are provided in insets573 and 575.

In the embodiment shown in FIG. 6 , as the optical signal 502 propagatesthrough the tapered portion 557 a of the upper waveguide core 554, thecoupling of the optical signals 502 between the upper and lower cores ofthe dual core waveguide 560 are substantially reduced relative to theweak coupling anticipated in the thin upper waveguide section 555 a.Optical signal processing in the arrayed waveguide structure 524 isaccomplished preferably in the single mode thick portion of the uppercore 554 in which the signals in the lower core 542 and the upper core554 are substantially decoupled.

In the embodiment shown in FIG. 6 a , the optical signal 502 traversesthe arrayed waveguide 524 substantially in the single mode upper core554 of the dual core waveguide 560 and exits at the right side of thedevice array as shown. In this embodiment shown in FIG. 6 , the opticalsignal 502 propagates through a second tapered portion 557 b of theupper waveguide 554 b in which the upper core thickness is reduced torecouple and expand the cross-sectional area of the optical signal 502in the lower waveguide 542. The progression of the optical signal 502 isshown in the inset 571 b in the portion of the dual core waveguide 560comprising the thick waveguide segment 554 b; in the inset 573 b for theportion of the dual core waveguide 560 comprising the thin waveguidesegment 555 b; and in the inset 575 b for the single mode fiber 582. Theembodiment shown in the top down view in FIG. 6 a is further illustratedin the cross section shown in FIG. 6 b within which the relative lateraldimensions of the various segments in the upper core 554, 555, 557 ofthe dual core waveguide 560 are aligned to highlight the specificfeatures from the top-down view of FIG. 6 a to the cross-section view ofFIG. 6 b . The top-down view and the cross-sectional view are not drawnto scale in FIGS. 5 and 6 , but rather are intended to highlight the keyfeatures of the structure in a vertically-expanded and more detailedformat in the cross-sectional view of FIG. 6 b . At the leftmost side ofthe device structure, the upper waveguide core in segment 555 a of thedual core waveguide, for example, is approximately 0.5 microns inthickness for a silicon oxynitride waveguide material. In an embodimentof the structure shown in FIG. 6 , that includes an arrayed waveguide524, the optical signal 502 enters the dual core waveguide structure 560from the left. The optical signal 502 enters the lower core 542 of thedual core waveguide 560, and transitions to the upper core 554 in thetransition region 557 a, in which the upper core 554 of the dual corewaveguide 560 is increased in thickness from 0.5 micron to 2 microns. Inembodiments, the dimension of the tapered section 557 a in the directionof signal propagation is in the range of 100-500 microns. In otherembodiments, the lateral dimension of the tapered section is 50 to 1000microns. In other embodiments, the lateral dimension of the taperedregion 557 is 150 microns. In yet other embodiments, the tapered section557 is such that the signal is substantially transferred from the lowercore 542 to the upper core 554. The rate of transition of the opticalsignal 502 between the upper and lower cores of the dual core waveguide560 can vary, and can depend in part on the refractive indices of theupper core and the lower core, and on the difference in the refractiveindex between the two cores. In the 2 micron thick portion of the uppercore 554, the optical signal 502 is substantially propagating in theupper core 554 and is substantially decoupled from the lower core 542.Demultiplexing of the optical signal 502 occurs in the arrayed waveguide524 which, in the embodiment shown, is substantially formed in the thickupper core 554 of the dual core waveguide 560. In embodiments, thethicker upper core 554 and the decoupling of the signal from the bottomcore 542 enable the signal 502 to propagate through the curved elementsof an optical device while maintaining at least adequate signalintensity for the subsequent decoding of the information from theincoming optical signal 502. In an exemplary embodiment, a decoding steppertains to the conversion of the optical signal to an electricalsignal, for example. In the portion of the arrayed waveguide devicestructure 524 that contains curvature in the optical pathway, theoptical losses are reduced with the increase in the thickness of theupper core 554 and the corresponding decoupling of the signal thatoccurs with the increased thickness of the upper core 554 of the dualcore waveguide 560.

As the optical signal 502 is demultiplexed in the arrayed waveguide 524(or other optical or optoelectrical device) in the embodiments shown inFIG. 4 , FIG. 5 , and FIG. 6 , the signal traverses the tapered region557 b of the dual core device structure 560. The thickness of the uppercore 554 of the dual core waveguide 560 transitions from theapproximately 2 micron thick region to a thinner 0.5 micron region inthis tapered portion 557 b. In embodiments, the dimension of the taperedsection 557 along the path of propagation is in the range of 100-500microns. In other embodiments, the dimension of the tapered sectionalong the path of propagation is 50 to 1000 microns. In otherembodiments, the dimension of the tapered region along the path ofpropagation 557 is 150 microns. In yet other embodiments, the dimensionof the tapered section 557 is such that the signal is substantiallytransferred from the lower core 542 to the upper core 554. The rate oftransition of the optical signal 502 between the upper and lower coresof the dual core waveguide 560 can vary, and can depend in part on therefractive indices of the upper core and the lower core, and on thedifference in the refractive index between the two cores. Beyond theoutput tapered region 557 b, the optical signal 502 is furtherpropagated in some embodiments, to individual output waveguides in someembodiments, output fibers 580 in other embodiments, and otherwaveguides, optical devices, or optoelectrical devices, for example inyet other embodiments.

In embodiments, the upper core 554 of the dual core waveguide 560 has ahigher refractive index than the lower core. In embodiments of the dualcore waveguide 560, the dual core structure 560 is fabricated from apolymer material or a dielectric material, or a combination of thesematerials. In other embodiments, the dual core waveguide 560 isfabricated from a stack of silicon nitride, silicon oxide, or siliconoxynitride layers, or a combination of these materials. In yet otherembodiments, the dual core waveguide 560 is fabricated from a compositestructure of multiple layers of silicon nitride, silicon oxide, andsilicon oxynitride layers. In other embodiments in which the dual corestructure 560 is a composite stack of layers, such as multiple siliconoxynitride layers, the upper core 554 of the dual core waveguide 560 hasa higher effective refractive index than the effective refractive indexof the lower core of the dual core waveguide 560. In this context, aneffective refractive index for the upper core 554 of the dual corewaveguide 560 refers to the refractive index of the grouped layerscomprising the upper waveguide 554.

In embodiments, the thickness of the lower core 542 of the dual corewaveguide 560 is on the order of the thickness of the core of themounted optical fiber 580. In some embodiments, the thickness of thelower core 542 of the dual core waveguide 560 is 10 microns. In otherembodiments, the thickness of the lower core is in the range of 5-15microns. In other embodiments, the thickness of the lower core 542 ofthe dual core waveguide 560 is thickness matched to the dimensions of anoptical signal 502 from an optoelectrical device mounted in proximity toa receiving portion of the lower core 542 of the dual core planarwaveguide 560. In yet other embodiments, the thickness of the lower core542 of the dual core waveguide structure is such that, when coupled toan optical fiber 580, an optical signal 502 transferred between theoptical fiber 580 and the lower core 542 of the dual core waveguide 560is substantially transferred so that at least a decodable signal ismaintained in optical device 524 or other optical or optoelectronicdevice.

The arrayed waveguides 524 shown in FIG. 5 and in FIG. 6 provideexemplary embodiments for a dual core waveguide structure 560. In otherembodiments, substrates 510 include optoelectronic devices in theoptical circuit. In yet other embodiments, the arrayed waveguide 524 isa multiplexer in which multiple optical signals propagate through aportion of the dual core waveguide 560 to the arrayed waveguide 524 andare combined before exiting the arrayed waveguide 524. In yet otherembodiments, a spot size converter is combined with one or more opticaldevices. In yet other embodiments, a spot size converter is formed fromthe combination of a thick lower core 542 and a single mode upper core554. In yet other embodiments, the stacking order of the dual corewaveguide structure 560 is reversed with the thin single core 554 of thewaveguide 560 formed closer to the substrate 510 and the thick core 542is formed above the thin single mode core 554 to form a spot sizeconverter or other optical device.

In embodiments, the thick core 542 of the dual core waveguide 560 has alower effective index of refraction than the effective index ofrefraction of the thinner upper core 554 to promote movement of theoptical signal from the region of lower refractive index to the regionof higher refractive index. In an exemplary embodiment, the effectiverefractive index of the thick lower core 542 of the dual core waveguide560 is 1.65 and the effective refractive index of the thin upper core554 is 1.7. In another embodiment, the effective refractive index of thethick lower core 542 of the dual core waveguide 560 is 1.68 and theeffective refractive index of the upper core 554 of the dual corewaveguide 560 is 1.71. Any of a range of refractive indices can be usedin embodiments, for the lower core 542 and the upper core 554 of thedual core waveguide. And any of a range of differences in the refractiveindices between the top core 554 and the bottom core 542 can be usedembodiments of the dual core waveguide 560. Preferably, the indices ofrefraction for dual core waveguides 560 fabricated from siliconoxynitride films are in the range of 1.4 to 2.0.

Referring to FIG. 7 , embodiments of a stacked dielectric film structure700, within which a dual core dielectric waveguide structure 760 isformed, is shown. The dual core dielectric waveguide structure 760 shownin the cross-sectional schematic in FIG. 7 a is a stack of dielectricfilms formed on a substrate 710. Optical devices such as waveguides andspot size converters for example, can be formed from the film structuresdescribed in FIG. 7 . The substrate 710 can be a substrate, aninterposer, a sub-mount, a sub-mount assembly, or other combination ofelements. The dual core waveguide 760, in embodiments, is a planarwaveguide structure. In some embodiments, the dual core waveguide 760 isa component in an optoelectronic circuit, photonic integrated circuit,electronic circuit, or substrate for which a planar waveguide iscombined with optical or optoelectronic devices, sensors,microelectromechanical device, or other device or devices. In anembodiment, the substrate 710 is silicon. In other embodiments, thesubstrate 710 is GaAs, InP, SiGe, SiC, or another semiconductor, orcombination of these or other commonly used semiconductor materials. Inyet other embodiments, the substrate 710 is aluminum nitride, aluminumoxide, silicon dioxide, quartz, glass, sapphire, or another ceramic ordielectric material. In yet other embodiments, the substrate 710 is ametal. And in yet other embodiments, the substrate 710 is a layeredstructure of one or more of a semiconductor, a ceramic, an insulator, adielectric, and a metal. It is to be understood that the substrate 710can be any material that provides a suitable mechanical support for thedual waveguide structure 760. It is to be further understood that asubstrate 710 with an interconnect layer that contains electrical linesand traces, separated with intermetal dielectric material, is asubstrate 710.

The dual core waveguide structure 760 includes a planar waveguidestructure formed on substrate 710. In an embodiment, the dual coreplanar waveguide structure 760 includes a buffer layer 730, bottomspacer layer 738, a repeating stack of silicon oxynitride films 742a-742 c that forms a first or lower core 742 of the dual core waveguide760, an intermediate spacer layer 750, a second stack or layer 754 thatforms a second or upper core 754 of the dual core waveguide 760, and anoptional top layer 758. Optional top layer 758 is a cladding layer insome embodiments. In some embodiments, bottom spacer layer 738 is acladding layer. In the discussion herein, descriptors such as top,bottom, upper, and lower are intended to provide relative positions ofthe various films in the structure and is not intended to limit theapplicability of the structure. Reversal of the order of the films, forexample, in the stack structure in embodiments with the substrate at thetop remains within the scope of the current invention. Additionally,reversal of the relative positions of the lower core 742 with that ofthe upper core 754 also remains within the scope of the currentinvention.

In embodiments, buffer layer 730 is one or more layers of silicondioxide or silicon oxynitride. In some embodiments, the buffer layer 730is a layer of silicon oxynitride. In an embodiment, the buffer layer 730is a silicon oxynitride layer, 5000 nm in thickness, with an index ofrefraction of 1.55. In other embodiments, the buffer layer 730 issilicon oxynitride with refractive index of 1.55 and is thicker than2000 nm. In other embodiments, the buffer layer 730 is a silicon dioxidelayer with a refractive index of approximately 1.445. In otherembodiments, the buffer layer 730 is a silicon dioxide layer with arefractive index of approximately 1.445 that is greater than 2000 nm inthickness. In yet other embodiments, the buffer layer 730 is a silicondioxide layer that is approximately 4000 nm in thickness and with arefractive index of approximately 1.445.

Buffer layer 730 can be a composite layer of one or more layers ofsilicon dioxide or silicon oxynitride with varying thicknesses that insome embodiments sum to greater than 4000 nm in total thickness.Similarly, the buffer layer 730, in some embodiments, can be a compositelayer of one or more layers with varying refractive index, that whencombined, provide a total thickness of greater than 4000 nm and aneffective refractive index in the range of 1.4 to 2.02. In otherembodiments, a buffer layer is thinner than 4000 nm. The buffer layer730 is an optional layer and is not required for all applications. In anembodiment in which a demultiplexer is fabricated from an arrayedwaveguide 524, for example, and the demultiplexing function isaccomplished in the upper core 754 of a dual core waveguide structure760, the use of a buffer layer 730 may not be required since the opticalsignal is guided from the lower waveguide to the upper waveguide in ashort distance from the interface between mounted optical fiber 580 andthe lower core 742 of the dual core waveguide structure 560, 760. Thebuffer layer 730, in some embodiments, provides a means for reducing themagnitude of the interaction between optical signals propagating withinthe waveguide cores 742, 754 in the dual waveguide structure 760 and theportions of the substrate 710 below the buffer layer 730. In otherembodiments, the buffer layer 730 provides a means for verticalalignment of one or more layers of the planar waveguide with otherfeatures on the substrate, interposer, or sub-assembly, such as opticalfibers and optoelectrical devices. As the thickness of the buffer layer730 is increased, the relative heights of planar waveguides 742,754within the dual waveguide structure 760 are also raised, for example,and the capability to change the relative height of the planar waveguidecores 742,754 relative to other devices on the substrate, interposer, orsub-mount assembly provides a useful means for alignment of the variousoptical, electrical, and optoelectrical components of a circuit orassembly within which the dual core waveguide structure 760 is utilized.In some embodiments, a buffer layer 730 is formed and a portion of thebuffer layer 730 is etched or otherwise removed to provide a featureheight that facilitates the vertical alignment of optical andoptoelectrical devices that are formed or mounted on the full or partiallayer of remaining buffer layer 730. In embodiments, a 5 micron thickbuffer layer may be cleared of overlayers and etched so that a remainingbuffer layer thickness of 2 micron is formed on the substrate 710. Inthese embodiments, a laser or other optoelectrical device is mounted onthe thinned buffer layer to align the emission axis or plane of thelaser or other optoelectrical device with the axis or plane of one ofthe waveguide cores in the dual core waveguide 760 to facilitate eitheremission of the laser into the aligned planar waveguide, or reception ofan optical signal from the aligned planar waveguide for detectors orother receptive optical devices.

Referring again to FIG. 7 a , and in particular to inset (i) in FIG. 7 a, spacer layer 738 is shown. In embodiments, spacer layer 738 is one ormore layers of silicon dioxide or silicon oxynitride. In otherembodiments, the spacer layer 738 is a single spacer layer 738 a ofsilicon oxynitride, 500 nm in thickness, with an index of refraction of1.55. In some embodiments, single spacer layer 738 is a layer 738 a of asingle material, such as silicon dioxide. In other embodiments, singlespacer layer 738 a is a layer of silicon oxynitride. In yet otherembodiments, single spacer layer 738 a is a layer of silicon oxynitridewith thickness in the range of 0 to 1000 nm. Although in someembodiments, a spacer layer 738 is included in the dual waveguidestructure 760, in some other embodiments, the spacer layer 738, can becombined with the buffer layer, can be made very thin, or is notincluded. In embodiments, the index of refraction of the spacer layer islower than the index of refraction of the lower core 742 of the dualcore waveguide 760.

Spacer layer 738 can be a composite spacer layer 738 b of one or morelayers of silicon oxynitride or silicon dioxide. In an embodiment,composite spacer layer 738 b includes two layers of silicon oxynitridewith thicknesses of 250 nm and with an effective refractive index ofapproximately 1.55. In some embodiments, the sum of the thicknesses ofthe two layers in composite spacer layer 738 b is in the range of 0 to1000 nm.

Similarly, the spacer layer 738 can be a composite layer 738 c of threeor more layers with the same or varying thicknesses and refractiveindices, that when combined, provide a total thickness in the range of 0nm to 1000 nm and an effective refractive index in the range ofapproximately 1.4 to approximately 2.02. Increasing the number of layersin the spacer layer 738, and in the overall stacked dielectric structure700 in general, can lead to a reduction in residual stresses and areduction in the optical signal losses in waveguides fabricated usingthe inventive film structures in some embodiments relative to waveguidestructures using fewer layers.

The combined thicknesses of the buffer layer 730 and the spacer layer738 in embodiments provide vertical spatial separation between the lowerwaveguide core 742 that is formed above the spacer layer 738 and thesubstrate 710. This vertical spatial separation is necessary in someembodiments, to reduce, minimize, or eliminate the interaction ofpropagating optical signals in the lower core 742 with the substrate710. The propagation of optical signals 502 with low optical lossthrough the lower waveguide core 742 requires some degree of confinementof the optical signal within the volume of the lower waveguide core withminimal interaction of the optical signals with the substrate 710 inembodiments for which the optical signals can be attenuated or otherwisereduced in interactions with the substrate material. Silicon and someother semiconductors, and metal layers in substrate interconnect layers,for example, can lead to significant attenuation of propagating opticalsignals in planar waveguides that are too close to the substrate toprovide adequate separation between the propagating signals and thesubstrate. The combined thicknesses of the buffer layer 730 and thespacer layer 738, in embodiments, provide spatial separation between thelayers of the inventive dual core dielectric waveguide structure 760 andthe substrate 710 and reduce the interaction of propagating opticalsignals with materials in the substrate that can lead to attenuation ofthe optical signals. Additionally, the spacer layer 738 is used in someembodiments to establish the relative height of the lower waveguide core742 and the upper waveguide core 754 that are formed above the spacerlayer 738. As is the case with the buffer layer 730, the thickness ofthe spacer layer 738 can be used as a means to provide alignment of theplanar waveguide cores 742, 754 that reside above the spacer layer 738with optical fibers, optoelectrical devices, optical devices, andelectrical devices formed, mounted, or placed in proximity to the planarwaveguide structure 760 to affect the mutual operation or couplingbetween the planar waveguides 742,754 and other circuit components.

In embodiments, lower core 742 of the inventive dual core planarwaveguide structure 760 is formed from a dielectric stack. In someembodiments, the dielectric film stack 742 is a layered structure ofsilicon oxynitride films.

Embodiments of the lower core 742 of the dual core planar waveguidestructure 760 are shown in inset (ii) in FIG. 7 a . In an embodiment,the lower waveguide core 742 of the dual waveguide structure 760 has arepeating stack 742 a of two dielectric films in which the constituentfilms within the repeating stack structure 742 a are of differingrefractive indices. The full thickness of the lower waveguide layer 742,in some embodiments, is obtained by the formation of a dielectric stackstructure in which the repeating stack 742 a is repeatedly deposited orotherwise formed to achieve the ultimate thickness of the layer 742. Thetwo dielectric films in the repeating stack 742 a are deposited in someembodiments using a chemical vapor deposition process or plasma enhancedchemical vapor deposition process, for example, and the process for thedeposition of the two films is repeated until the full thickness of thewaveguide layer 742 is obtained. Differences in the refractive indicesare obtained in some embodiments, from changes in the stoichiometriccomposition of the films. In embodiments, the changes in thestoichiometry of the films in the repeating film structure 742 a isaccomplished with changes in the process conditions used in thedeposition of the films comprising the repeating film structure 742 a.In an exemplary embodiment, the repeating stack structure 742 a includesa first film 743 of 900 nm of silicon oxynitride with an index ofrefraction of 1.6 and a second film 744 of 50 nm of silicon oxynitridewith an index of refraction of 1.7. In another embodiment, the repeatingstructure 742 a includes a first film 743 of 40 nm of silicon oxynitridewith an index of refraction of 1.7 and a second film 744 of 500 nm ofsilicon oxynitride with an index of refraction of 1.65. In yet anotherembodiment, the repeating structure 742 a includes a first film 743 of60 nm of silicon oxynitride with an index of refraction of 1.7 and asecond film 744 of 500 nm of silicon oxynitride with an index ofrefraction of 1.65. It is to be understood that the order of the firstfilm 743 and the second film 744 in embodiments can be reversed andremain within the scope and spirit of the invention. It should also benoted that the index of refraction of a film structure 742 comprised ofmultiple films will provide an effective index of refraction for theoverall stack 742 that depends at least partly on the thickness of theindividual layers in the film structure 742, the index of refraction ofeach of the layers in the film structure 742, and on the thicknesses andindices of refraction of the interfacial layers between the individuallayers in the film structure, among other factors Additionally, theeffective index of refraction of the overall film structure of the lowerwaveguide core 742 will depend on the method of formation or depositionof the constituent films and the stoichiometry of the films, among otherfactors, such as impurities and dopants present in the film structure.

In other embodiments, the lower waveguide core 742 has a repeating stack742 b of more than two dielectric films in which the constituent films745-747 within the repeating structure 742 b are of differing refractiveindices, and in some embodiments, of the same or differing thicknesses.In an exemplary embodiment, repeating stack 742 b includes a first film745 of 400 nm of silicon oxynitride with an index of refraction of 1.6,a second film 746 of 500 nm of silicon oxynitride with an index ofrefraction of 1.65, and a third film 747 of 50 nm of silicon oxynitridewith an index of refraction of 1.7. Repeating stack 742 b in embodimentsis repeated multiple times to provide the ultimate thickness of thefirst core 742 of the dual core waveguide structure 760.

In yet other embodiments, repeating stack 742 c of the lower waveguidecore 742 includes more than three layers for which the index ofrefraction, and in some embodiments the thickness, for the constituentlayers is varied to achieve the total film thickness of the lowerwaveguide core structure 742. In the inset (ii) in FIG. 7 a , filmstructure 742 c shows embodiments for which the repeating stack 742 c inthe lower core 742 is comprised of a first layer 748 i, a second layer748 ii, a third layer 748 iii, and additional layers through 748 n,where n is the nth layer of a multilayer repeating structure 742 ccomprising the repeating stack 742 c. The repeating stack 742 c isformed and repeated, wholly or in part, as necessary until an ultimatethickness is provided for the lower core 742 of the dual core waveguide760. In an exemplary embodiment in which a large number of individuallayers 748 i-748 n is used to form the lower core 742 of the dual corewaveguide 760, for which the overall thickness of the lower waveguidecore 742 is 9 microns, a repeating stack of 45 constituent layers of 100nm each (in which case n=45) is used in which the repeating structure742 c need only be repeated twice to achieve the overall thickness. Inyet other embodiments, the repeating structure 742 c of the lowerwaveguide core dielectric stack 742 has a layered film structure thatdoes not repeat because the total number of constituent films in therepeating stack provides the overall film thickness for the lower core742 of the dual core waveguide 760 (for example, for 100 nm thick films,n=90). By contrast in an embodiment, for example, in which the repeatingfilm structure 742 a has two constituent films (n=2) with a combinedthickness of 600 nm, the stack is repeated 15 times to reach an overallthickness of 9 microns for the dielectric film stack 742. Multiplelayers in the repeating structure 742 a-742 c can facilitate a reductionin stress in the lower waveguide core 742, and ultimately in the overalldielectric film structure 700 in comparison to bulk films. That is, thenumber of layers that comprise the lower core 742 of the dual waveguidestructure 760 facilitates a means for controlling the stress levels inthe overall structure 700. The programmability of modern productionequipment for the deposition of dielectric films has enabled wideflexibility in the number and thickness of films that can be programmedinto a process recipe for the formation of film structures. Thiscapability of modern film-deposition production equipment enables theformation of both large numbers of films, as in embodiments forrepeating structure 742 c comprised of layers 748 n for which n can be alarge number, and of large numbers of repetitions of multilayer filmsets as in repeating structure 742 a, comprised of a pair of films thatis repeatedly deposited or otherwise formed a large number of times toachieve the required film thickness of the bottom waveguide core 742. Alarge number of films, for example, can be on the order of 10's ofdifferent layers, or more. In some embodiments, for example, thethickness and stoichiometry in every layer can be different from everyother layer, and for structures with very thin layers, on the order of10 nm for example, the number of films can be on the order of a hundredlayers for a 10 micron thick waveguide core 742.

In other embodiments of the lower core 742 of the dual core waveguide760, the repeating film structure is a composite structure of multiplerepeating stacks. In embodiments, repeating structures comprised ofmultiple two-layer repeating structures 742 a are used to form the lowerwaveguide core 742. For example, the repeating film structure 742 a foran embodiment in which the first layer 743 is 900 nm and the secondlayer 744 is 50 nm has a total two-layer repeating stack thickness 742 aof 950 nm, and when combined with a second two-layer repeating filmstructure 742 a for an embodiment in which the first layer 743 is 800 nmand the second layer 744 is 60 nm, for a combined thickness of 860 nm,the resulting overall combined film thickness, when repeated 5 times,will provide a total thickness for the lower core 742 of 9050 nm ((900nm+50 nm)+(800 nm+60 nm)×5=9050 nm)). In embodiments, variations in themakeup of the lower core 742 provides a high level of flexibility inminimizing the resulting stress levels in the lower waveguide core 742and the overall dual waveguide structure 700, and for controlling theeffective index of refraction of the resulting lower waveguide core 742.In an embodiment, the effective index of refraction for the lowerwaveguide core 742 comprised of multiple two-layer repeating structures742 a is 1.6. In another embodiment, the effective index of refractionfor the lower waveguide core 742 comprised of multiple two-layerrepeating structures 742 a is 1.65. In yet other embodiments, theeffective index of refraction for the lower waveguide core 742 comprisedof multiple two-layer repeating structures 742 a is less than therefractive index of the upper core 754 of the dual core waveguide 760.And in yet other embodiments, the effective index of refraction for thelower waveguide core 742 comprised of multiple two-layer repeatingstructures 742 a is in the range of approximately 1.4 to 2.0.

In other embodiments, the repeating two-layer stack 742 a is combinedwith a three-layer stack 742 b to provide a combined five-layerrepeating stack. In yet other embodiments, other combinations of one ormore two-layer repeating stacks 742 a are combined to provide a combinedmultilayer repeating stack. In yet other embodiments, one or moretwo-layer repeating stacks 742 a are combined with one or morethree-layer repeating stacks 742 b to provide a combined multilayerrepeating stack. And in yet other embodiments, one or more two-layerrepeating stacks 742 a are combined with one or more three-layerrepeating stacks 742 b and one or more multilayer stacks 742 c toprovide a combined multilayer repeating stack. In embodiments,variations in the makeup of the lower core 742 provides a high level offlexibility in minimizing the resulting stress levels in the lowerwaveguide core 742 and the overall dual waveguide structure 700, and forcontrolling the effective index of refraction of the resulting lowerwaveguide core 742. In an embodiment, the effective index of refractionfor the lower waveguide core 742 comprised of combinations of multiplerepeating structures 742 a-742 c is 1.6. In another embodiment, theeffective index of refraction for the lower waveguide core 742 comprisedof multiple combinations of repeating structures 742 a-742 c is 1.65. Inyet other embodiments, the effective index of refraction for the lowerwaveguide core 742 comprised of multiple repeating structures 742 a-742c is less than the refractive index of the upper core 754 of the dualcore waveguide 760. And in yet other embodiments, the effective index ofrefraction for the lower waveguide core 742 comprised of multiplerepeating structures 742 a-742 c is in the range of 1.4 to 2.0.

Generally, the overall thickness of the lower core 742 in embodimentsprovides low optical loss for optical signals transmitted through theresulting dual core waveguide structure 760. The multilayer structureensures low stress in the resulting film structure and enables thickwaveguides (2000 nm to 25000 nm) to be formed. Waveguide structures 742are thus sufficiently thick to enable transmission of the opticalsignals with little interaction of the transmitted optical signals withthe substrate, interaction levels that could lead to undesiredattenuation of the transmitted signals. Additionally, first core 742 ofthe dual core waveguide 760 is thickness-matched in some embodiments tothe cores of mounted optical fibers 580 that are mounted on, or arelocated in proximity to, the substrate, interposer, or sub-mountassembly. Alternatively, in embodiments, in which the first core 742 ofthe dual core waveguide 760 is aligned to receive signals from mountedoptoelectrical devices, the thickness of first core 742 of the dual corewaveguide 760 is thickness-matched to substantially receive a signalfrom the optoelectrical devices from which the signals are received.

It is to be understood that the thickness, the number of films, and therefractive index for the films in dielectric stack 760 can vary andremain within the scope of the current invention. The refractive indexof silicon oxynitride films can vary in the range of 1.4 to 2.02. As theconcentration of nitrogen in deposited silicon oxynitride films isminimized, the refractive index approaches the index of refraction ofsilicon dioxide, 1.445. Conversely, as the concentration of oxygen isminimized in the deposited films, the refractive index approaches theindex of refraction of silicon nitride, 2.02. The index of refractioncan thusly be varied in the range of 1.445 to 2.02 by varying thestoichiometric concentration of silicon, oxygen, and nitrogen in each ofthe deposited layers, and in the overall film structure of the lowerwaveguide core 742. In embodiments, the index of refraction for theconstituent films 743, 744 in the repeating dielectric film stack 742 a,for example, are varied in the range of 1.445 to 2.02 to produce thickfilm structures of 2000 to 25000 nm, or greater, and that provide lowstress and low optical signal losses, in the dielectric film stacks 700.And in yet other embodiments, a distribution of the index of refractionof the films in the lower waveguide core 742 is provided. In anembodiment, for example, the first layer in the lower waveguide core 742is deposited with a low value, 1.6 for example, and the index ofrefraction for subsequently deposited layers in the stack above thisfirst layer is progressively increased until the midpoint or approximatemidpoint in the thickness of the lower waveguide core 742 is reached. Ator near the midpoint of the film structure of the lower waveguide core742, the index of refraction is greater than the first layer, say 1.7for example. Then, in subsequently deposited films in the lowerwaveguide core 742, above the midpoint, the index of refraction isprogressively decreased until the top layer of the bottom waveguide core742 structure is reached. In this example, the profile of the index ofrefraction is varied, such that a profiled structure in the index ofrefraction is formed. Other profiles can also be formed in otherexemplary embodiments. Alternatively, the thickness of the films in thestructure can be varied, and in yet other embodiments, both thethickness and the index of refraction can be varied.

In an embodiment, the lower core 742 includes a repeating stack 742 awith a first layer 743 of silicon oxynitride with thickness of 60 nm andan index of refraction of 1.7 and a second layer 744 of siliconoxynitride with thickness of 500 nm and an index of refraction of 1.65.Repeating dielectric stack structure 742 a is repeated in an embodiment13 times for a total thickness for dielectric film stack 742 of 7280 nm.It is to be understood that the total number of repeating film stacks742 a can vary. In some embodiments, the number of repeating film stacks742 a is three to twenty. In some other embodiments, the repeating filmstack 742 a is such to produce a total dielectric film structure 742that in some embodiments is greater than 2000 nm in thickness and insome embodiments less than 25000 nm. In yet other embodiments, the totaldielectric film structure 742 is in the range of 8000 to 12000 nm. Inyet other embodiments, the number of repeating film stacks is two ormore and the thickness of the lower core 742 is greater than 2000 nm andless than 25000 nm. In yet other embodiments, the lower core 742 of thedual core planar waveguide structure 760 is thickness matched tosubstantially receive an optical signal from a mounted optical fiber 580or an optoelectrical device that is used to deliver a signal to thelower core 742 of the dual core waveguide structure 760.

In some embodiments, the thickness for the first film 743 is in therange of 5 nm to 1000 nm. In some other embodiments, the thickness ofthe second film 744 is in the range of 5 nm to 1000 nm. In these andother embodiments, the thickness of the dielectric film structure 742,which is the sum of the thicknesses of the repeating film structures 742a, is greater than 2000 nm in thickness. In yet other embodiments, thethickness of the sum of the repeating film structures 742 a is in therange of 4000 to 10000 nm. In yet other embodiments, the thickness ofthe sum of the repeating film structures 742 a is thickness-matched orsubstantially thickness-matched to the core of a mounted optical fiber580 or to the size of an optical signal 502 from an optoelectrical oroptical device from which the lower core of the planar waveguide 742receives all or a portion of the signal.

It is to be understood that the number of films, the film thicknesses,the refractive indices, and the resulting composition of the films inthe lower core 742 can be varied and remain within the spirit and scopeof the inventive dual core waveguide structure 760. In embodiments, theuse of a multiplicity of films provides low stress and low optical lossfor signals transmitted through waveguides that are fabricated from theinventive dielectric stack structure in the dual core waveguidestructure 760. In this regard, in some embodiments, an initial repeatingfilm structure 742 a is used for two or more of the films in thedielectric stack 742, and a different repeating film structure 742 a isused for another two or more films in the same dielectric film structureto produce the lower core 742. It is to be further understood that aninitial repeating film structure 742 a can be used for two or more ofthe films in the lower core 742, and a different repeating filmstructure 742 a, can be used for another two or more films, and then anynumber of additional repeating film structures 742 a with the same ordifferent repeating film structures can be used for two or moreadditional films in the film structure 742 and remain within the scopeand spirit of the embodiments. In the foregoing discussion, thevariations in the first film 743 and second film 744 can be producedwith one or more variations in the refractive index, the thickness, andthe composition or stoichiometry of the films.

It is also to be understood that in some embodiments, first film 743 inthe repeating film structure 742 a can include one or more films andremain within the scope of the invention. In an embodiment, first film743 in repeating film structure 742 a, for example, is 500 nm inthickness with a refractive index of 1.7. In another embodiment, firstfilm 743 includes a first part that is 250 nm in thickness with arefractive index of 1.7 and a second part that is 250 nm in thicknesswith a refractive index of 1.65. In yet another embodiment, the firstfilm 743 in the repeating film structure 742 a has a refractive index of1.68 with a first partial thickness that is 250 nm and a second partialthickness that is deposited in a separate process step from the first,for example, and that is also 250 nm in thickness for a combinedthickness of 500 nm for the two partial films of the first film 743 ofrepeating film structure 742 a.

In some embodiments, the first film 743 has a graded refractive index orstoichiometric composition. Gradations in the composition of the firstfilm 743 of the repeating film structure 742 a, for example, remainwithin the scope of the current invention. In some embodiments, therefractive index varies through part or all of the thickness of thefirst film 743. Similarly, in some embodiments, the stoichiometriccomposition varies through part or all of the thickness of the firstfilm 743. Variations in the refractive index or the stoichiometriccomposition of the first film 743 within the thickness of this filmremain within the scope of the current invention.

It is also to be understood that in some embodiments, second film 744 inthe repeating film structure 742 a can include one or more films andremain within the scope of the invention. In an embodiment, second film744 in repeating film structure 742 a, for example, is 500 nm inthickness with a refractive index of 1.7. In another embodiment, secondfilm 744 includes a first part that is 250 nm in thickness with arefractive index of 1.7 and a second part that is 250 nm in thicknesswith a refractive index of 1.65. In yet another embodiment, the secondfilm 744 in the repeating film structure 742 a has a refractive index of1.68 with a first partial thickness that is 250 nm and a second partialthickness that is deposited in a separate process step from the first,for example, that is also 250 nm for a combined thickness of 500 nm forthe two partial films of the second film 744 of the repeating filmstructure 742 a.

In some embodiments, the second film 744 has a graded refractive indexor stoichiometric composition. Gradations in the composition of thesecond film 744 of the repeating film structure 742 a, for example,remain within the scope of the current invention. In some embodiments,the refractive index varies through part or all of the thickness of thesecond film 744. Similarly, the stoichiometric composition variesthrough part or all of the thickness of the second film 744. Variationsin the refractive index or the stoichiometric composition of the secondfilm 744 within the thickness of this film remain within the scope ofthe current invention.

In yet other embodiments, the extent of the gradations in the first orsecond film can be such that the first or second film need not beincluded. That is, in some embodiments, a single layer repeatingstructure 742 a is formed by grading the stoichiometry of the film 743and then repeating the deposition of the graded layer 742 a until thefull thickness of the lower waveguide core is formed. And in yet otherembodiments, one or more graded single layers 743 with one or moregradation in one or more film properties comprise the repeatingstructure 742 a-742 c to form the full thickness of the lower core 742of the dual waveguide structure 760.

In some embodiments, repeating structure 742 has an unequal number offirst layers 743 and second layers 744. In some embodiments, repeatingstructure 742 a includes a first layer 743 positioned between two secondlayers 744.

In embodiments, top spacer layer 750 is one or more layers of siliconoxide or silicon oxynitride as shown in inset (iii) FIG. 7 b . In someembodiments, single spacer layer 750 a is a layer of one type ofmaterial, such as silicon dioxide. In other embodiments, single spacerlayer 750 a is a layer of silicon oxynitride. In yet other embodiments,the single spacer layer 750 a is a layer of silicon oxynitride withrefractive index of 1.55 and with a thickness of 500 nm. In yet otherembodiments, the single spacer layer 150 a is a layer of siliconoxynitride with refractive index of 1.6 and with a thickness of 500 nm.In yet other embodiments, single spacer layer 750 a is a layer ofsilicon oxynitride with thickness in the range of 0 to 1000 nm. Althoughin some embodiments, a spacer layer 750 a is included in the structure,in some other embodiments, the spacer layer 750 can be combined withother layers above or below the spacer layer, can be made very thin, oris not included.

In some embodiments, spacer layer 750 is a composite spacer layer 750 bof one or more layers of silicon oxynitride or silicon dioxide. In anembodiment, composite spacer layer 750 b includes two layers of siliconoxynitride with thicknesses of 250 nm and with an effective refractiveindex of approximately 1.55. In some embodiments, the sum of thethicknesses of the two layers in composite spacer layer 750 b is in therange of 0 to 1000 nm.

Similarly, in some embodiments, the spacer layer 750 is a compositelayer 750 c of three or more layers with the same or differentthicknesses and refractive indices, that when combined, provide a totalthickness in the range of 0 nm to 1000 nm and an effective refractiveindex in the range of 1.4 to 2.02. The index of refraction of the spacerlayer, in some embodiments, is 1.6. In some other embodiments, the indexof refraction of the spacer layer is less than the index of refractionof the lower core. In other embodiments, the index of refraction ofspacer layer 750, relative to the index of refraction of the lower core742 of the dual core waveguide 760, is such that the optical signal isweakly coupled between the upper core 754 and lower core 742 of the dualwaveguide structure. In some embodiments, the index of refraction ofspacer layer 750, or the index of refraction of the composite layer forembodiments with a composite spacer layer 750 c, is greater than theindex of refraction of the lower core 742 to promote the coupling ortransfer of the optical signal from the lower core 742 of the dual corewaveguide 760 to the upper core 754. Certain design parameters of thedual core waveguide 760 will affect the selection of the relativeindices of refraction of the lower core 742, the spacer 750 between theupper core 754 and lower core 742, and the upper core 754 of the dualcore waveguide 760, as well as the other layers in the overall structureof the dual core planar waveguide structure 760.

The thickness of the spacer layer 750 can vary, and in some embodiments,provides mechanical spacing required to align the upper waveguide core754 to optoelectrical or optical devices in proximity to the upperwaveguide core 754. In embodiments, the index of refraction is lowerthan that of the lower waveguide core 742 and the upper waveguide core754. In other embodiments in which one or more of the lower waveguidecore 742 and the upper waveguide core 754 is a composite structurecomprised of multiple layers, the index of refraction of the spacerlayer 750 is lower than that of the lower waveguide core 742 and theupper waveguide core 754. In some embodiments, the spacer layer 750 is acladding layer for either or both of the lower waveguide core 742 andthe upper waveguide core 754.

In some embodiments, the upper core 754 of the dual core waveguide 760is a single layer of silicon oxynitride. In some embodiments, the uppercore 754 of the dual core waveguide 760 is a single layer of siliconoxynitride with an index of refraction in the range of 1.4 to 2.02. Insome embodiments the upper core 754 of the dual core waveguide 760 isformed of a multiplicity of films as described herein for the formationof the lower core 742 of the dual core waveguide 760. In embodiments,the upper core 754 of the dual core waveguide 760 is limited inthickness to maintain stable single mode propagation of the transmittedoptical signal. The thickness of the upper waveguide core 754 that isrequired to provide stable single mode propagation can vary with thewavelength of the optical signals.

Generally, as the thickness of a planar waveguide is increased, thelikelihood for a propagating optical signal in the dual waveguidestructure to transition between multiple available and allowed modesincreases. That is to say, that while the number of availablepropagation modes for a small waveguide can be limited in someconfigurations to a single propagation mode, the number of availablepropagation modes for larger waveguides increases as the cross-sectionalarea of a waveguide is increased. And, as the cross-sectional area ofthe larger waveguides is increased, and additional propagation modesbecome possible, the likelihood of a transition to one of thesealternative propagation modes increases. It is therefore beneficial tolimit the thickness of the upper waveguide core 754 to dimensions thatare sufficiently large to enable single mode propagation yet not sothick as to enable propagation in undesirable higher order propagationmodes. The range of thickness of the upper core 754 of the dual corewaveguide 760, in embodiments, is such that the propagation mode islimited to that which is required by the photonic integrated circuitwithin which the dual core waveguide is implemented. In otherembodiments, the range of thickness is limited to that which willprovide single mode propagation of the optical signals for which thephotonic integrated circuit is designed or utilized. The thickness ofthe upper waveguide core 754 that is required to provide stable singlemode propagation can vary with the wavelength of the optical signals.

In embodiments, the upper core 754 of the dual core waveguide 760 is inthe range of thickness from 0.5 to 2.5 microns for optical signals inthe range of 1300 to 1600 nm. In other embodiments, the thickness of theupper core 754 of the dual core waveguide 760 is in the range of 0.25 to5 microns. The actual thickness of the upper core 754 of the dual corewaveguide 760 can vary depending on the specific application for whichthe dual core waveguide 760 is utilized, the distance of the pathwaythrough which the optical signals must travel, and the amount ofcurvature in the optical waveguide core 754, for example. Inembodiments, the index of refraction and the thickness of the upper core754 of the dual core waveguide 760, are selected to enable single modepropagation for the wavelength of the propagating optical signal 502.

Additionally, in embodiments, the tapered region 557 of the dual corewaveguide 560, along with or independently of other design elements, isused to aid in directing the optical signal 502 between the upper core554 and lower cores 542 of the dual core waveguide 560 as required, andthe thickness of the upper core 754 in the tapered region 557 will varyas described herein.

Referring again to FIG. 7 , and to insets (ii) and (iii) in FIG. 7 a ,some embodiments of the upper core 754 are shown. In some embodiments,upper core 754 of the dual core waveguide 760 is one or more layers ofsilicon dioxide or silicon oxynitride. In other embodiments, the uppercore 754 a is a single layer of silicon oxynitride, 2000 nm inthickness, with an index of refraction of 1.7. In some embodiments,upper core 754 a is a single layer of a single material, such as silicondioxide. In yet other embodiments, the upper core 754 a is a layer ofsilicon oxynitride with refractive index of 1.68 with thickness of 1800nm. In yet other embodiments, upper core 754 a is a layer of siliconoxynitride with thickness in the range of 500 to 2500 nm.

Upper core 754 in some embodiments is a composite layer 754 b of one ormore layers of silicon oxynitride or silicon dioxide. In an embodiment,composite upper core layer 754 b includes two layers of siliconoxynitride with thicknesses of 1000 nm and with an effective refractiveindex of approximately 1.70. In some embodiments, the sum of thethicknesses of each of the two layers in composite upper core layer 754b is in the range of 1 to 1250 nm.

Similarly, the upper core layer 754 can be a composite layer 754 c ofthree or more layers with the same or differing thicknesses andrefractive indices, that when combined, provide a total thickness in therange of 500 nm to 2500 nm and an effective refractive index in therange of 1.4 to 2.02. Multiple layers in the upper core 754 of the dualwaveguide structure can lead to a reduction in residual stresses in thedual waveguide structure 760, and therefore, by increasing the number oflayers in some embodiments, the level of stress in the overall dualwaveguide film structure 700 can be reduced or minimized relative tobulk films.

In embodiments, a stack of dielectric films forms an upper core 754 ofthe inventive dual core planar waveguide structure 760 through whichoptical signals can propagate. In some embodiments, the upper core 754is a layered structure of silicon oxynitride films.

In some embodiments, the upper core 754 of the dual core waveguide 760is a composite structure of silicon oxynitride films. In someembodiments, a stack of layers is used to form the upper core 754similar to embodiments comprised of the stacks of layers described forthe lower core 742 as in inset (ii) in FIG. 7 a . The use of multiplelayers such as repeating film structures 742 a, 742 b, and 742 c canenable the formation of thick lower waveguide core structures 742 withlow residual stress levels and with low optical loss characteristics.The low optical loss characteristics are of particular importance forplanar waveguide applications. Similar approaches to those used in theformation of the lower waveguide 742 can be utilized in the formation ofthe upper waveguide 754 as described herein.

In an embodiment, the dielectric stack 754 has a repeating stack 754 dof two dielectric films in which the constituent films within therepeating stack structure 754 d are of differing refractive indices.Differences in the refractive indices can occur, for example, fromchanges in the stoichiometric composition of the films. In embodiments,the changes in the stoichiometry of the films in the repeating filmstructure 754 d is accomplished with changes in one or more of theprocess conditions that include such parameters as gas flow, gasmixtures, power level, temperature, and pressure, among others, used inchemical vapor deposition processes for the formation of the films inthe repeating film structure 754 d. In an exemplary embodiment, therepeating stack structure 754 d includes a first film 743 of 90 nm ofsilicon oxynitride with an index of refraction of 1.65 and a second film744 of 50 nm of silicon oxynitride with an index of refraction of 1.7.In another embodiment, the repeating structure 754 d includes a firstfilm 743 of 40 nm of silicon oxynitride with an index of refraction of1.7 and a second film 144 of 50 nm of silicon oxynitride with an indexof refraction of 1.65. In yet another embodiment, the repeatingstructure 154 d includes a first film 743 of 60 nm of silicon oxynitridewith an index of refraction of 1.7 and a second film 744 of 100 nm ofsilicon oxynitride with an index of refraction of 1.65. It is to beunderstood that the order of the first film 743 and the second film 744in embodiments can be reversed and remain within the scope and spirit ofthe invention. Multiple layers of the repeating stack 754 d aredeposited or otherwise formed to provide the ultimate thickness of theupper waveguide core 754.

In another embodiment, the dielectric stack 754 is formed from arepeating stack 754 e of three two dielectric films in which theconstituent films 745-747 within the repeating structure 754 e are ofdiffering refractive indices, and in some embodiments, of the same ordiffering thicknesses. In an exemplary embodiment, repeating stack 754 eincludes a first film 745 of 40 nm of silicon oxynitride with an indexof refraction of 1.7, a second film 746 of 50 nm of silicon oxynitridewith an index of refraction of 1.68, and a third film 747 of 20 nm ofsilicon oxynitride with an index of refraction of 1.65. In otherembodiments, other combinations of film thicknesses and indices ofrefraction for each film in the repeating stack 754 e are used. Multiplelayers of the repeating stack 754 e are deposited or otherwise formed toprovide the ultimate thickness of the upper waveguide core 754.

In yet other embodiments, the repeating stack 754 f of the upper core754 includes more than three layers for which the index of refraction ineach layer differs relative to adjacent layers to achieve the total filmthickness of the upper core 754 of the dual core waveguide structure760. In the inset (ii) in FIG. 7 a , embodiments are shown for which therepeating stack 754 f in the upper core 754 is comprised of a firstlayer 748 i, a second layer 748 ii, a third layer 748 iii, andadditional layers through 748 n, where n is the nth layer of amultilayered upper waveguide core 754 comprised of multiple layers ofthe repeating stack 754 f. The repeating stack 754 f is formed andrepeated, wholly or in part, as necessary until an ultimate thickness isprovided for the upper core 754 of the dual core waveguide 760. In anexemplary embodiment in which a large number of individual layers 748i-748 n is used to form the upper core 754 of the dual core waveguide760, and for which the overall thickness of the upper waveguide core 754is 2.4 microns, a repeating stack of 12 layers, each 100 nm in thickness(in which case n=12) is used in which the repeating structure 754 f needonly be repeated twice to achieve the overall upper waveguide corethickness of 2.4 microns.

In yet other embodiments, the repeating structure 754 f of the upperwaveguide core 754 has a layered film structure that does not repeatbecause the combined thickness of the films in the repeating structure754 f provides the overall film thickness for the upper core 754 of thedual core waveguide 760. In an exemplary embodiment, a repeating stackis comprised of 24 films, each 100 nm in thickness, and for which thetotal thickness of the repeating stack 754 f corresponds to thethickness of the overall film stack of 2.4 microns (for 100 nm thickfilms, n=24). By contrast in an embodiment, for example, in which therepeating film structure 754 f has two constituent films (n=2) with acombined thickness of 600 nm, the stack is repeated 4 times to reach anoverall thickness of 2.4 microns for the upper waveguide film stack 754.Multiple layers in a repeating structure 754 d-754 f can facilitate areduction in stress in the upper waveguide core 754, and ultimately inthe overall dielectric film structure 700 in comparison to bulk films.That is, the number of layers that comprise the upper core 754 of thedual waveguide structure 760 facilitates a reduction in the stresslevels in the overall structure 700. The programmability of modernproduction equipment for the deposition of dielectric films has enabledwide flexibility in the number and thickness of films that can beprogrammed into a process recipe for the formation of film structures.This capability of modern film-deposition production equipment enablesthe formation of both large numbers of films, as in embodiments forrepeating structure 754 f comprised of layers 754 i through 754 n, forwhich n is a large number, and of large numbers of repetitions ofmultilayer film sets as in repeating structure 754 b, comprised of apair of films that is repeatedly deposited or otherwise formed a largenumber of times to achieve the required film thickness of the upperwaveguide core 754.

Further clarification is provided with a comparison of embodimentscomprised of a two-layer repeating structure 754 d and a six-layerrepeating structure 754 f. In embodiments, for example, for which therepeating film structure 754 d has two constituent films of 300 nm eachfor a combined thickness of 600 nm, the stack must be repeated 3 timesto obtain an overall thickness of 1.8 microns for the upper waveguidecore 754. (2 layers/repeating structure×3 repetitions=6 total layers inwaveguide 754); also (2×300 nm=600 nm×3 repetitions=1.8 microns.)) Bycomparison, in embodiments for which the overall thickness of the upperwaveguide is also 1.8 microns, a repeating stack 754 f of sixconstituent layers of 150 nm each requires that the overall repeatingstructure need only be repeated twice to achieve the targeted upper corethickness of 1.8 microns ((6 layers/repeating structure×2 repetitions=12total layers in waveguide 754); also, (6×150 nm=900 nm×2 repetitions=1.8microns.))

In yet other embodiments, the repeating structure 754 f of upper core754 has a layered film structure that does not repeat because the totalnumber of films in the repeating stack provides the overall filmthickness for the full thickness of the upper core 754. For example, inan embodiment for which the upper waveguide core is 2 microns, arepeating stack 754 f comprised of 20 layers of silicon oxynitride thatare 100 nm in thickness each, does not require any repetitions toprovide the targeted thickness of the upper core 754.

In some embodiments of the upper core 754 of the dual core waveguide760, the repeating film structure 754 f is a composite structure ofrepeating stacks. In embodiments with a repeating stack structure, theoverall thickness of the upper core 754 is the combined thickness of therepeating stack 754 d, 754 e, 754 f multiplied by the number of timesthat the repeating stack is repeated. For example, the repeating filmstructure 754 d for an embodiment in which the first layer 743 is 100 nmand the second layer 744 is 50 nm has a total repeating stack thicknessof 150 nm and when repeated 10 times, the resulting combined filmthickness for the upper core stack 754 is 1500 nm ((100 nm+50nm)×10=1500 nm)). Similarly, in another embodiment, the repeating filmstructure 754 d, which has a first layer 743 that is 40 nm with arefractive index of 1.7, and which has a second layer 744 that is 50 nmin thickness with a refractive index of 1.65, has a combined thicknessof 90 nm, and when repeated 20 times, has a resulting combined filmthickness for an upper core thickness of 1800 nm ((50 nm+40 nm)×20=1800nm)).

Generally, the overall thickness of the upper core 754 is such that thepropagation of optical signals in the upper core 754 is limited to asingle propagation mode. The use of multiple film layers in the uppercore 754 of the dual core waveguide 760 can provide low stress in theresulting film structure and enables thick dual core waveguidestructures 700 to be formed.

It is to be understood that the thickness, the number of films, and therefractive index for the films in the upper core 754 can vary and remainwithin the scope of the current invention. The refractive index ofsilicon oxynitride films can vary in the range of 1.4 to 2.02. As theconcentration of nitrogen in deposited silicon oxynitride films isminimized, the refractive index approaches the index of refraction ofsilicon dioxide, 1.445. Conversely, as the concentration of oxygen isminimized in the deposited films, the refractive index approaches theindex of refraction of silicon nitride, 2.02. The index of refractioncan thusly be varied in the range of 1.445 to 2.02 by varying thestoichiometric concentration of silicon, oxygen, and nitrogen in thedeposited films. In embodiments, the index of refraction for theconstituent films 743, 744 in the upper core 754, for example, arevaried in the range of 1.445 to 2.02 to produce film structures ofapproximately 500 to 2500 nm, and that provide low stress and lowoptical signal losses. In some embodiments, the index of refraction, andin some embodiments the effective index of refraction, in the upper core754 is greater than the index of refraction or effective index ofrefraction of the lower core 742.

In other embodiments, the upper core 754 of the dual core waveguide 760includes a repeating stack 754 d with a first layer 743 of siliconoxynitride with thickness of 60 nm and an index of refraction of 1.7 anda second layer 744 of silicon oxynitride with thickness of 50 nm and anindex of refraction of 1.68. Repeating stack structure 754 d is repeatedin an embodiment 20 times for a total thickness for the upper core 754of 2200 nm. It is to be understood that the total number of repeatingfilm stacks 754 d can vary. In some embodiments, the number of repeatingfilm stacks 754 d is three to twenty. In some other embodiments, therepeating film stack 754 d is such to produce a total thickness of theupper core 754 that in some embodiments is greater than 500 nm inthickness and in some embodiments less than 3000 nm. In yet otherembodiments, the total thickness of the upper core 754 is in the rangeof 800 to 2000 nm. In yet other embodiments, the number of repeatingfilm stacks is two or more and the thickness of the upper core 754 isgreater than 100 nm and less than 5000 nm.

In some embodiments, the thickness for the first film 743 is in therange of 5 nm to 1000 nm. In some other embodiments, the thickness ofthe second film 744 is in the range of 5 nm to 1000 nm. In these andother embodiments, the thickness of the upper core 754, which is the sumof the thicknesses of the repeating film structures 754 d-754 f, isgreater than 2000 nm.

In some embodiments, an initial repeating film structure is used for twoor more of the films in the upper core 754, and a different repeatingfilm structure is used for another two or more films in the samedielectric film structure to produce the overall repeating filmstructure 754 f of the upper core 754. It is to be further understoodthat an initial repeating film structure can be used for two or more ofthe films in the upper core 754, a different repeating film structure,can be used for another two or more films in the same dielectric filmstructure of the upper core 754, and then any number of additionalrepeating film structures with the same or different repeating filmstructures can be used for two or more additional films to producecomposite repeating structure 754 f for the upper core dielectric filmstructure 754 and remain within the scope and spirit of the embodiments.In the foregoing discussion, the variations in the first film 748 andsecond film 749 can be produced with one or more variations in therefractive index, the thickness, and the composition or stoichiometry ofthe films.

It is also to be understood that in some embodiments, first film 743 inthe repeating film structure 754 d can include one or more films andremain within the scope of the invention. In an embodiment, first film743 in repeating film structure 754 d, for example, is 50 nm inthickness with a refractive index of 1.7. In another embodiment, firstfilm 743 includes a first part that is 250 nm in thickness with arefractive index of 1.7 and a second part that is 250 nm in thicknesswith a refractive index of 1.65. In yet another embodiment, the firstfilm 743 in the repeating film structure 754 d has a refractive index of1.68 with a first partial thickness that is 250 nm and a second partialthickness that is deposited in a separate process step from the first,for example, and that is also 250 nm in thickness for a combinedthickness of 500 nm for the two partial films of the first film 743 ofrepeating film structure 754 d.

In some embodiments, the first film 743 has a graded refractive index orstoichiometric composition. Gradations in the composition of the firstfilm 743 of the repeating film structure 754 d, for example, remainwithin the scope of the current invention. In some embodiments, therefractive index varies through part or all of the thickness of thefirst film 743 used in the formation of the repeating stack structuresfor the upper core 754. Similarly, in some embodiments, thestoichiometric composition varies through part or all of the thicknessof the first film 743. Variations in the refractive index or thestoichiometric composition of the first film 743 within the thicknessesof these films remains within the scope of the current invention for theupper core 754 of the dual core waveguide structure 760.

It is also to be understood that in some embodiments, second film 744 inthe repeating film structure 754 a can include one or more films andremain within the scope of the invention. In an embodiment, second film744 in repeating film structure 754 d, for example, is 500 nm inthickness with a refractive index of 1.7. In another embodiment, secondfilm 744 includes a first part that is 250 nm in thickness with arefractive index of 1.7 and a second part that is 250 nm in thicknesswith a refractive index of 1.65. In yet another embodiment, the secondfilm 744 in the repeating film structure 754 d has a refractive index of1.68 with a first partial thickness that is 250 nm and a second partialthickness that is deposited in a separate process step from the first,for example, that is also 250 nm for a combined thickness of 500 nm forthe two partial films of the second film 744 of the repeating filmstructure 754 d.

In some embodiments, the second film 744 has a graded refractive indexor stoichiometric composition. Gradations in the composition of thesecond film 744 of the repeating film structure 754 d, for example,remain within the scope of the current invention. In some embodiments,the refractive index varies through part or all of the thickness of thesecond film 744. Similarly, the stoichiometric composition variesthrough part or all of the thickness of the second film 744. Variationsin the refractive index or the stoichiometric composition of the secondfilm 744 within the thickness of this film remain within the scope ofthe current invention for the upper core 754 of the dual core waveguide760.

In some embodiments, the repeating structure used in the formation ofthe upper core 754 of the dual core waveguide 760 has an unequal numberof first layers 743 and second layers 744. In some embodiments,repeating structure 742 includes a first layer 743 positioned betweentwo second layers 744.

Optional top layer 758 is one or more layers of a dielectric materialsuch as silicon dioxide, for example. Optional layer 758, in someembodiments is a layer of silicon dioxide with thickness of 200 nm and arefractive index of 1.445. In some embodiments, the film thickness ofthe top layer is in the range of 0 to 5000 nm. In some embodiments,silicon oxynitride is used in the optional top layer 758. Inembodiments, the index of the refraction of the optional layer 758 islower than the index of refraction of the adjacent underlayer 754. Insome embodiments, the upper layer is a waveguide cladding layer. In yetother embodiments, another dielectric material or combination ofmaterials such as silicon nitride, aluminum nitride, or aluminum oxideis used. In yet other embodiments, no optional top layer 758 is providedin the stack 700. In yet other embodiments, the optional layer 758 is aportion of a hermetic seal that is formed on the planar waveguidestructure 700.

Referring to FIG. 7 b , a three-dimensional perspective view of theexemplary structure shown in FIG. 7 a is shown. The arrow in FIG. 7 bshows the typical direction of propagation for optical signals travelingalong the axis of a planar waveguide structure formed from embodimentsof the stacked film structures as described herein.

In FIG. 8 , a cross-sectional schematic illustration is shown for anexemplary embodiment of the inventive dual waveguide structure 800. FIG.8 shows some specific features and layers in an embodiment of the dualwaveguide structure 800. A thick buffer layer 830 is shown over asubstrate 810. In this exemplary embodiment, the substrate 810 is, forexample, a silicon substrate. In other embodiments, the same or othersubstrates are used. In an exemplary embodiment, the buffer layer 830 isa layer of silicon dioxide that is 5500 nm in thickness with anapproximate index of refraction of 1.5. In other embodiments, siliconoxynitride or another material is used for the buffer layer 830. In yetother embodiments, other thicknesses and other refractive indices areused for the buffer layer 830. Above the buffer layer 830 in FIG. 8 is aspacer layer 838. In the exemplary embodiment, the spacer layer 838 is alayer of silicon oxynitride, 500 nm in thickness with an index ofrefraction of 1.6. Above the spacer layer 838 is lower waveguide core842 consisting of repeating structure 842 a. The repeating structure 842a is comprised of a first layer 843 and a second layer 844. In anexemplary embodiment, the first film 743 is a silicon oxynitride filmwith a thickness of 900 nm and index of refraction of 1.6 and a secondsilicon oxynitride film 744 with a thickness of 50 nm and an index ofrefraction of 1.7. The repeating structure 842 a is repeated ten timesto form the lower core 842 of the exemplary embodiment of the dual corewaveguide 860 with a thickness of 9.5 microns. Above the lower core 842of the dual core waveguide 860 is a spacer layer 850. In the exemplaryembodiment, the spacer layer 850 is a layer of silicon oxynitride withan index of refraction of 1.6 and a thickness of 200 nm. In otherembodiments, other spacer layer thicknesses are used with either higheror lower indices of refraction. Above the spacer layer 850 in theexemplary embodiment, is the upper waveguide core layer 854 of the dualcore waveguide 860. In the exemplary embodiment, the upper core 854 is asilicon oxynitride layer with an index of refraction of 1.7 and athickness of 2000 nm. Above the upper waveguide core 854 in theexemplary embodiment shown in FIG. 8 is top layer 858. In this exemplaryembodiment of a dual core waveguide structure 800, the top layer is asilicon oxynitride cladding layer, 900 nm in thickness, with an index ofrefraction of 1.6. In other embodiments, other thicknesses of the toplayer are used with the same or other indices of refraction.

In FIG. 9 , flow diagram 990 is shown for the formation of the inventivedual core waveguide 700 for some embodiments. In the flow chart 990, afirst core of the dual core waveguide structure 700 is a lower waveguidecore 742 and this first core 742 is formed 992 on substrate 710. In someembodiments, one or more of a buffer layer 730 and a bottom spacer layer738 is formed on the substrate 710 prior to the formation of the lowercore 742. A spacer layer formation step 994 follows the lower coreformation step 992. In some embodiments, the spacer layer 750 influencesthe magnitude of the signal coupling between the lower core 742 and theupper core 754 of optical signals propagating in the dual core waveguideportion 760 of the waveguide structure 700. An upper core 754 is formedin upper core formation step 996 following the spacer layer formationstep 994 to form a dual core waveguide 760 in some embodiments. Thelower waveguide core 742, the spacer layer 750, and the upper core 754form the primary structural components of the dual core waveguidestructure 760. Specific details of the lower waveguide core 742, thespacer layer 750, and the upper waveguide core 754 of the dual corewaveguide structure in embodiments are provided herein in the discussionof FIG. 7 and elsewhere.

Referring to FIG. 10 , a flow diagram 1090 for the formation ofembodiments of the inventive dual core waveguide structure 700 in someembodiments that includes a tapered region 557, to facilitate theformation of an optical device such as a spot size converter, forexample, is provided. In the flow diagram 1090, a first core of the dualcore waveguide structure 700 is formed in first core formation step 1092on a substrate 710. In embodiments, the first core of the dual corewaveguide 700 is lower waveguide core 742. In some embodiments, one ormore of a buffer layer 730 and a spacer layer 738 are formed on thesubstrate 710 prior to the formation of the bottom core 742 of the dualcore waveguide 760. In some embodiments, a spacer layer 750 is formed onthe first core 742 of the dual core waveguide structure 700 in spacerlayer formation step 1094. The spacer layer 750, in some embodiments,can influence the magnitude of the coupling between the lower waveguidecore 742 and the upper waveguide core 754 in the dual core waveguidestructure 700. An upper core 754 of the dual core waveguide 700 isformed on the spacer layer 750 in upper core formation step 1096.Optional layers such as a top cladding layer 758 as described herein,are provided in some embodiments. Taper formation step 1098 of the uppercore 754 of the dual core waveguide 700 follows the spacer layerformation 1096. The taper 557 in the upper waveguide core 754 enablesthe transition of optical signals from the lower waveguide core 742 tothe upper core 754 of the dual core waveguide 700 in device structuressuch as a spot size converter. In regions in which the upper core 754 isthinned, to approximately 0.5 microns in some embodiments, the upperwaveguide core 754 is weakly coupled to the lower core 742, and signalstraveling in the dual core waveguide structure propagate substantiallyin the thick bottom core 742 of the dual waveguide structure. In thetapered portion 557 of the upper waveguide core 554, 754 of the dualcore waveguide 700, in which the thickness of the upper core waveguide754 increases in thickness along the path of the optical signalspropagating in the optical circuit, as shown for example in FIG. 5 , thecoupling of the optical signal 502 between the lower waveguide core 742and the upper waveguide core 754 is decreased to the extent thatcoupling between the upper core 754 and lower waveguide core 742 iseliminated, minimized, or reduced to an extent that signal processing issubstantially independent of the lower core 742. The tapering of theupper core 754 of the dual core waveguide 700 can be an increasingtaper, in which the upper waveguide thickness is increased, or adecreasing taper, in which the upper waveguide thickness is decreased,or both. Increasing or decreasing the thickness of the upper core 754 ofthe dual core waveguide 700 along the path of the optical signalinfluences the magnitude of the coupling between the two waveguide coresfor optical signals propagating in the dual core waveguide structure700.

One or more portions of the upper waveguide 754 is increased inthickness, and one or more portions of the upper waveguide 754 isdecreased in thickness along the optical pathway in some embodiments asthe requirements for the optical circuits that utilize the dual corewaveguide necessitate.

Embodiments in which the part of the upper waveguide structure at one ormore portions of the upper waveguide 754 is increased in thickness, andpart of the upper waveguide structure at one or more portions of theupper waveguide 754 is decreased in thickness include optical devicessuch as spot size converters often used at the entry points and exitpoints of arrayed waveguides and other optical devices, for example. Thedual core waveguide structure 800 with tapered sections 557 in the uppercore 754 can function as a spot size converter. Spot size converters arefrequently used in the conversion of the optical signal size from alarge waveguide, such as an optical fiber, for example, to a smallerwaveguide, such as a single mode planar waveguide.

Referring to FIG. 11 , a flow diagram 1190 for the formation of theinventive dual core waveguide 700 in some embodiments that includes atapered region 557, to facilitate the formation of an optical devicesuch as a spot size converter, for example, is provided. In the flowdiagram 1190, a first core of the dual core waveguide structure 700 isformed in first core formation step 1192 on a substrate 710. Inembodiments, the first core of the dual core waveguide 700 is lowerwaveguide core 742, and is formed from a layered structure of siliconoxynitride films. In some embodiments, one or more of a buffer layer 730and a spacer layer 738 are formed on the substrate 710 prior to theformation of the bottom core 742 in bottom core forming step 1192 of thedual core waveguide 700. A spacer layer 750 of silicon oxynitride isformed on the first core 742 of the dual core waveguide structure 700 inspacer layer formation step 1194. The spacer layer 750, in someembodiments, influences the extent of the coupling between the lowerwaveguide core 742 and the upper waveguide core 754 of the dual corewaveguide 700. An upper waveguide core 754 of the dual core waveguide700 is formed on the spacer layer 750 in upper core formation step 1196.In some embodiments, the upper core 754 is a silicon oxynitride layer.In other embodiments, the upper core 754 is a polymer layer. In yetother embodiments, the upper core 754 is a dielectric material such assilicon dioxide. Optional layers such as a top cladding layer 758 asdescribed herein, are formed on the upper waveguide core 754 in someembodiments. Taper formation step 1198 of the upper waveguide core 754of the dual core waveguide 700 follows the upper core formation step1196. The tapered section 557 in the upper waveguide core 754 enablesthe transition of optical signals from the lower waveguide core 742 tothe upper core 754 of the dual core waveguide 700. In regions in whichthe upper core 754 is thinned, to approximately 0.5 microns in someembodiments, optical signals propagating in the upper waveguide core 754can be weakly coupled to the lower waveguide core 742. In these regionswith the thinned upper waveguide core 755, optical signals traveling inthe dual core waveguide structure propagate substantially in the thickbottom core 742 of the dual waveguide structure. In the tapered portion557 of the upper core 554, 754 of the dual core waveguide 700, in whichthe thickness of the upper core waveguide 754 increases in thicknessalong the path of optical signals propagating in the optical circuit, asshown in FIG. 5 , the coupling of the optical signal between the lowerwaveguide core 742 and the upper waveguide core 754 is decreased to theextent that coupling between the upper core 754 and lower waveguide core742 is eliminated, minimized, or reduced to an extent that signalprocessing is substantially independent of the lower core 742. Thetapering of the upper core 754 of the dual core waveguide 700, in thedirection of signal propagation, can be an increasing taper, in whichthe upper waveguide thickness is increased, or a decreasing taper, inwhich the upper waveguide thickness is decreased, or both. Increasing ordecreasing the thickness of the upper core 754 of the dual corewaveguide 700 along the path of the optical signal influences themagnitude of the coupling between the two waveguide cores for opticalsignals propagating in the dual core waveguide structure 700. For thinupper core layers of silicon oxynitride, of approximately 0.5 micronsfor example, optical signals propagating in the dual core structure 700are weakly coupled in some embodiments. As the thickness is increased,to approximately 2 microns in the case of silicon oxynitride films, forexample, optical signals that are transitioned through the taperedsection 557 become substantially decoupled. Embodiments in which thepart of the upper waveguide structure at one or more portions of theupper waveguide 754 is increased in thickness, and part of the upperwaveguide structure at one or more portions of the upper waveguide 754is decreased in thickness include optical devices such as spot sizeconverters often used at the entry points and exit points of arrayedwaveguides and other optical devices, for example. The dual corewaveguide structure 700 with tapered sections 557 in the upper core 754can function as a spot size converter. Spot size converters arefrequently used in the conversion of the optical signal size from alarge waveguide, such as an optical fiber, for example, to a smallerwaveguide, such as a single mode planar waveguide.

Embodiments of the dual core waveguide 700 are enabled with thickwaveguide formation steps such as those described herein for the siliconoxynitride materials. The dual waveguide structure 700 provides a meansfor substantially coupling a thick planar waveguide formed on asubstrate to an optical fiber 580 or to an optical or optoelectricaldevice mounted in proximity to the planar waveguide. Optical signals aretransferred from optical fibers, in some embodiments for example, to thethick planar lower cores. Signals propagating in the lower cores of thedual core structure 700 are weakly coupled to the upper core. Opticalsignal processing of the propagating signal is facilitated with themovement of the signal from the thick planar multimode lower core 742 ofthe dual core waveguide 700 to the single mode upper core 754 of thedual core waveguide 700 particularly for optical waveguides that requirebends or curvature in the planar waveguide. Arrayed waveguides, forexample, require curved pathways as a means for separating an opticalsignal comprised of multiple wavelengths into the constituent opticalsignals. In thin, single mode waveguides, the optical signalspropagating through the portions of the optical devices in which thewaveguides have curvature can be subject to signal degradation and loss.This loss in signal integrity in arrayed waveguides, as well as in otheroptical devices that have curved pathways, is reduced or eliminated inembodiments with the upper and lower cores of the dual core waveguide700 allow for substantially decoupled signal propagation. Decouplingoccurs in embodiments for thick upper waveguide cores of approximatelytwo microns in cases for which silicon oxynitride is used to form thedual waveguide structure 700. The improvement in signal integrity isanticipated from the decoupling of the signals propagating in the twowaveguide cores as the upper core 754 is increased in thickness fromapproximately 0.5 microns to approximately 2 microns in someembodiments.

In embodiments, the dual core dielectric film structures can be formedusing chemical vapor deposition (CVD) methods, for example. Chemicalvapor deposition (CVD) and plasma enhanced chemical vapor deposition(PECVD), for example, enable the formation of the dielectric stackstructures described in FIG. 7 . In embodiments, the formation of thedual core waveguide structures in FIG. 7 requires the capability tocontrol the levels of stress in the stack structure to enable therequired thicknesses for the bottom core 742 to substantially match tothe diameter of aligned single mode fibers 580, for example.

The capability to control the measured film stress using PECVD in theformation of dielectric film stacks of silicon oxynitride is shown inFIG. 12 . In FIG. 12 a , the measured film stress is shown for a rangeof thicknesses for dielectric film stacks that were fabricated usingPECVD. FIG. 12 a shows that embodiments of a composite film structurecan provide stress levels of approximately 20 MPa and less forembodiments as thick as approximately 18 microns. Low stress in filmsthicker than 18 microns are also possible. The effectiveness of theapproach of utilizing a stacked structure of silicon oxynitride, orother dielectric materials, for forming thick waveguides in the range ofpractical interest is demonstrated with the data in FIG. 12 a .Additional supporting data are provided in FIG. 12 b , in which thevariation in the measured stress for a set of films in which therefractive index was varied is shown. For the measured films from whichthe data is provided in FIG. 12 b , the refractive index was varied byvarying the gas flows in a PECVD silicon oxynitride deposition process.Each data point in FIG. 12 b is obtained from a deposited film in whichthe gas mixture in the deposition process was varied in a controllableway relative to other films from which the data points in FIG. 12 b wereobtained. The data show, that by varying the gas flow, and hence therefractive index, the measured stress level in the deposited films canbe varied over a wide range, and in a controllable manner. The variationin the refractive index, an easily measurable parameter, shows how thevariation of a measurable characteristic of a film can affect themeasured stress. The capability to control the measured stress isnecessary in compensating for stresses in the film structure to producean overall film stress for the stack structure. Stress levels ofapproximately 20 MPa or less are generally considered acceptable inthick planar waveguide structures. In embodiments, the approach ofproviding films with both tensile and compressive stresses into theoverall film structure is utilized rather than an approach of attemptingto deposit a single layer, for example, at the zero-stress intersectionof the fitted line shown in FIG. 12 b . Overall stress levels incomposite film structures are kept within specified levels by varyingthe type of stress (“+” or “−” values of film stress as shown in FIG. 12b ) and the thickness of each of the films as described in embodimentsin FIG. 7 to alleviate the accumulation of stress for the thick filmstructures required for thick waveguides.

Although conceivably possible, relatively low stress levels are verydifficult to achieve in films of a single thick layer of material suchas silicon dioxide or silicon oxynitride. The introduction of themultilayer film structure, as described in FIG. 7 , enables thickstructures with low stress to be formed, however. In FIG. 12 b , themeasured stress levels for deposited silicon oxynitride films are shownfor films of various refractive indices. As shown, the refractive indexis a convenient means for assessing variations in film properties fordeposited films. The capability to achieve control of the stress in theindividual films over a wide range, allows for the fabrication of thethick dielectric film structures (1000-25000 nm, and greater) requiredfor use as planar waveguides. In embodiments, stress levels arecontrolled in planar waveguide structures to minimize deformation of thesubstrates upon which the thick dielectric stacks are deposited and toprevent delamination of the films, for example. Using the capability tovary the stress in the films, stacks of layers can be formed in whichstress in the growing film structure can be compensated for withstresses that counter the stress in the film. For example, if the stressat a given thickness is compressive, then a film with tensile stress canbe deposited to compensate for the residual stress in the film. The useof the compensating films in the film stack as required to meet theoverall residual stress specification, <20 MPa for example, offers analternative to the growth of a single bulk film with low stress, andallows for the formation of very thick dielectric stack structures of upto 10 microns and greater. In addition to magnitude and type of stressin a given film, proper selection of the thickness of each of thedeposited layers provides an additional means for controlling theultimate stress in the overall stack structure 700.

In addition to the low stress levels required for optical communicationapplications, fabricated waveguides must also exhibit low opticallosses. Optical loss is a measure of the reduction in the strength of anoptical signal as it propagates through a waveguide or optical device.For practical applications, desirable loss specifications are typicallyless than 1 dB/cm for a planar waveguide, for example.

Referring to FIG. 13 , measured optical losses from embodiments of thedielectric stack structures are shown. Optical signal losses forpractical use in planar waveguide structures of less than approximately1 dB/cm are desirable. FIG. 13 a shows optical loss levels of less thanapproximately 1 dB/cm for a number of films for which the measuredeffective refractive indices are shown. In addition to the properties ofthe dielectric stack structure itself, the buffer layer also has aninfluence on the measured optical signal loss. FIG. 13 b shows how thethickness of the buffer layer in some embodiments affects the measuredoptical losses. As the thickness of the buffer layer is increased inthese embodiments, the resulting optical losses are reduced to values ofmuch less than 1 dB/cm. The data in FIGS. 12 and 13 were obtained fromsingle core waveguide structures fabricated from thick dielectric filmstructures that were not dual core waveguide structures, although themethods for controlling the stress in the upper waveguide core aresimilar to those used in the formation of single core waveguides.

Referring to FIG. 14 , steps in the formation of embodiments of thedielectric films and film structures are provided. In FIG. 14 a ,forming step 1493 is a process of forming embodiments of the dielectricstack 700 of silicon oxynitride films at low temperature having lowstress and low optical loss. Low temperature in FIG. 14 a refers to thetemperature of the deposition of the films used in the fabrication ofthe dielectric stacks, namely less than 500° C. in some embodiments, andin other embodiments, less than or approximately equal to 300° C. Lowstress in FIG. 14 a refers to stress levels in the deposited films infilm structure 700 of less than or equal to approximately 20 MPa, eithercompressive or tensile. Low optical loss in FIG. 14 a refers to opticallosses in embodiments of deposited dielectric film stacks 700 of lessthan approximately 1 dB/cm. The forming step 1493 provides for theformation of thick structures of dielectric silicon oxynitride filmswith low stress, and suitable for use in the transmission of opticalsignals with low loss.

Referring to FIG. 14 b , the forming steps 1495 in embodiments for whicheach individual layer in the dielectric stack 700 of silicon oxynitridefilms is deposited at low temperature, and with low stress and lowoptical loss is shown. Low temperature in FIG. 14 b refers to thetemperature of the deposition of the films used in the fabrication ofthe dielectric stacks, namely less than 500° C. in some embodiments, andin other embodiments, less than or equal to 300° C. Low stress in FIG.14 b refers to stress levels in the deposited films of less than orequal to approximately 20 MPa, either compressive or tensile. Stresslevels of less than 20 MPa in deposited films ensure minimal substratedeformation and reduce the likelihood that the films will delaminate.Low optical loss in FIG. 14 b refers to optical losses in embodiments ofdeposited dielectric film stacks 700 of less than approximately 1 dB/cm.Forming step 1495 provides for the formation of thin composite films ofdielectric silicon oxynitride deposited sequentially at low temperaturesof less than 500° C. to form the thick dielectric stack structures 700with low stress, and suitable for use in the transmission of opticalsignals with low loss.

Referring to FIG. 14 c , steps in the formation of dual core planarwaveguides from a forming step 1497 and a patterning step 1499 are shownfor some embodiments. Formation of the individual dielectric films andthe dielectric film structures 1497 for the inventive stack structure700 are shown that include the formation of a dielectric stack ofsilicon oxynitride films on a substrate 710 with a stack structure thatincludes a buffer layer 730, one or more optional bottom spacer layers738, a repeating stack of one or more dielectric layers to form bottomwaveguide core 742, an optional spacer layer 750, an upper waveguidecore 754 comprised of a one or more layers, and an optional top layer758. Embodiments for the forming of the dielectric film and filmstructures 1497 utilize one or more of forming step 1493 and formingstep 1495. Patterning step 1499 is combined in embodiments with formingstep 1497 on the resulting dielectric stack to form one or more planarwaveguide structures on a substrate from the dielectric stack structures700. Patterning steps can include the use of established photoresistpatterning processes, in which photosensitive layers are used eitherdirectly as a means for transferring a pattern with subsequent dry orwet etch processing, or via a hard mask in which the photoresist isfirst used to transfer a pattern to a hard mask layer that is then usedto transfer the waveguide pattern from the hard mask layer to thedielectric stack layer. Processes for photoresist patterning andsubsequent wet and dry etching of film structures are well establishedfor those skilled in the art of dielectric film patterning techniques.Patterning steps can also include the steps required to thin and taperthe upper waveguide core 754 as described herein to form, for example, aspot size converter.

Typical deposition processes for dielectric films used in semiconductordevice manufacturing include chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), metalorganic chemical vapordeposition (MOCVD), physical vapor deposition (PVD), atomic layerdeposition (ALD), among others.

In embodiments, deposition of the dielectric film structure 700 isaccomplished using PECVD. In other embodiments, deposition of theinventive dielectric film structure 700 is accomplished using one ormore of the thin film deposition techniques that include chemical vapordeposition (CVD), plasma enhanced chemical vapor deposition (PECVD),metalorganic chemical vapor deposition (MOCVD), physical vapordeposition (PVD), and atomic layer deposition (ALD).

Process parameters such as the process pressure, substrate temperature,and process power levels, as well as the selection of precursor gasesand the flows of these and other gases used in the process, amongothers, can each affect the resulting properties of the deposited films.In the fabrication of nitrogen-containing films such as silicon nitrideand silicon oxynitride, for example, precursors such as ammonia gas(NH₃), a source of nitrogen, and silane (SiH₄), a source of silicon, arecombined in a plasma environment to facilitate the formation of the thinsilicon nitride films. The ammonia and silane can be further combinedwith one or more oxidizing gases such as oxygen (O₂), nitric oxide (NO),and nitrous oxide (N₂O) to incorporate oxygen into the growing films toform silicon oxynitride. The properties of the silicon nitride and thesilicon oxynitride films can be affected by the specific selection ofgases in the process mixture and the ratio of the specific gas flows inthe mixture. Residual impurities, such as hydrogen, can also affect theproperties of the deposited films, particularly through the formation ofnitrogen-hydrogen bonds in the deposited films. In embodiments, aprocess is provided to fabricate dielectric film structure 700 in whichone or more nitrogen precursors that do not contain hydrogen are used inthe formation of thin films of silicon nitride and silicon oxynitride toreduce the residual hydrogen concentration in the deposited films.

In embodiments, a plasma enhanced chemical vapor deposition (PECVD)system, as shown for example in the schematic drawing in FIG. 15 , isused to deposit silicon oxynitride film structures 700. PECVD processesare most typically performed under vacuum conditions with a selection ofprecursor gases specific to the types of films that are targeted fordeposition. Substrates are most typically heated to elevatedtemperatures. In the configuration shown in FIG. 15 , radio frequency(RF) power is capacitively coupled to the electrode opposite to that onwhich the substrate resides during the process. In the PECVD systemdepicted in FIG. 15 , substrates reside on the lower electrode duringprocessing, and the RF power is applied to the upper electrode. Inembodiments in which the power is applied to the upper electrode, thesubstrate is not subjected directly to high energy ion bombardment aswould be anticipated if the substrate were to be placed on the RFpowered electrode.

In other embodiments, the electrode upon which the substrate residesduring the deposition process is powered. Powering of the substrate canlead to energetic ion bombardment during processing, and can contributeto the level of residual stress in deposited films.

In an embodiment of the present invention, multilayer dielectric filmstructure 700 is formed using PECVD technology with a process chemistrythat utilizes hydrogen-free nitrogen and oxygen precursors to yieldplanar waveguides with low optical loss and low stress. PECVD technologyis widely used in semiconductor and optical device fabrication. Much ofthe equipment used in the manufacturing of semiconductor devices can beused in the fabrication of optical devices and the use of the term“semiconductor device fabrication” herein is intended to include opticaldevice fabrication.

In embodiments, the dielectric film stack 700 is deposited onto asubstrate using PECVD technology at temperatures in the range of200−500° C. In other embodiments, the substrate temperature duringdeposition of the dielectric film structure 700 is in the range ofapproximately 250-400° C. In yet other embodiments, the dielectric filmstack 700 is deposited onto a substrate using PECVD technology attemperatures of approximately 300° C. And in yet another embodiment, thedielectric film stack 700 is deposited at or approximately 300° C. usinga gas mixture of silane, nitrogen, and nitrous oxide. In yet anotherembodiment, the dielectric film stack is deposited using a gas mixtureof silane, nitrogen, and oxygen. In yet another embodiment, thedielectric film stack 700 is deposited at or approximately 300° C. usinga gas mixture of silane, nitrogen, and nitric oxide.

In general, a gas mixture used in the deposition of silicon oxynitridefilms must contain at least the primary stoichiometric constituents orelements of the film, namely, silicon, oxygen, and nitrogen. Control ofthe deposited dielectric film properties is achieved, in part, with thecontrol of the gas flows and mixture ratios of the constituent precursorgases in the deposition system. In an embodiment, for example, silanegas (SiH₄) is used as a precursor to provide the silicon, nitrogen gas(N₂) is used as a precursor for nitrogen, and oxygen gas (O₂) is used asa precursor for oxygen. In embodiments, the independent control of oneor more of the gas flows, the ratios of the gases, and the partialpressures of these three gases in the deposition system can provide ameans for independent control of the ratio of the three elements, namelySi, N, and O, in the deposited films. Other PECVD system parameters canalso affect the stoichiometry of the resulting films.

In embodiments, silane gas, one of the most commonly used precursors forthe deposition of silicon and silicon-containing thin films in epitaxialand chemical vapor deposition processes in semiconductor devicefabrication, is provided as a source for silicon in the deposited filmstructure 700. In other embodiments, one or more of a silicon precursorthat includes dichlorosilane, trichlorosilane, methylsilane, silicontetrachloride, chlorosilane, dichlorosilane, and trichlorosilane isutilized as a silicon precursor in the deposited dielectric filmstructure 700.

In embodiments, a silicon-containing precursor gas that may or may notcontain hydrogen, is combined with a hydrogen-free precursor gas fornitrogen, and a hydrogen-free precursor gas for oxygen to produce theoxynitride layers in the dielectric stack structure 700. In otherembodiments, silane (SiH₄) is used as the silicon precursor in the PECVDdeposition of silicon oxynitride films. In yet other embodiments, silaneis used as the silicon precursor and is mixed with hydrogen-freeprecursors for nitrogen and oxygen to form films of silicon oxynitride.

In addition to the source of silicon in one or more of the precursors inthe deposition gas mixture, precursors for embodiments with siliconoxynitride in the dielectric film structure 700 include one or more ofan oxygen-containing precursor, a nitrogen-containing precursor, and aprecursor that contains both oxygen and nitrogen. Precursors thatcontain both oxygen and nitrogen elements in embodiments include nitrousoxide, nitric oxide, nitrogen dioxide, dinitrogen tetraoxide, andmixtures of these gases, for example. In other embodiments, anitrogen-containing precursor that does not contain oxygen is used incombination with an oxygen-containing precursor that does not containnitrogen. In an embodiment, nitrogen gas (N₂) is the nitrogen-containingprecursor and is combined with oxygen gas (O₂) as the oxygen-containingprecursor, and further combined with silane or anothersilicon-containing precursor described herein in a plasma enhancedchemical vapor deposition system. In yet other embodiments, anitrogen-containing precursor such as nitrogen gas is combined with oneor more precursors that contain both oxygen and nitrogen, such asnitrous oxide and nitric oxide, or a mixture thereof, and furthercombined with silane or another silicon-containing precursor asdescribed herein in a plasma enhanced chemical vapor deposition systemto form the silicon oxynitride layers in dielectric stack 700. In yetother embodiments, an oxygen-containing precursor such as oxygen gas iscombined with one or more precursors that contain both oxygen andnitrogen, such as nitrous oxide and nitric oxide, and further combinedwith silane or another silicon-containing precursor in a plasma enhancedchemical vapor deposition system or other deposition system as describedherein to form dielectric stack 700. Oxygen-containing precursorsinclude atomic oxygen, molecular oxygen, ozone, carbon monoxide, andcarbon dioxide.

And in yet other embodiments, one or more of argon, helium, neon, xenon,nitrogen, and oxygen is added to the gas mixtures described herein inembodiments as a diluent. Diluents are often utilized in semiconductordeposition processes to slow the deposition rate, to enhance theignition of the plasma in plasma-based processes, to improve depositionuniformity, and to alter or modify the energy absorption profile of thegaseous species in the plasma, among other potential benefits. Additionof one or more diluents listed herein to the processes described hereinremains within the scope of the current invention.

In embodiments, the use of hydrogen-free, nitrogen and oxygen precursorsin the deposition of the dielectric film stacks 700 yields low stresssilicon oxynitride film structures from which waveguides can be formedthat exhibit low optical signal loss.

In other embodiments, low stress, low optical loss silicon oxynitridefilm structures are formed using plasma enhanced chemical vapordeposition with a gas mixture that contains one or more of each of asilicon-containing precursor, a nitrogen-containing precursor, and anoxygen-containing precursor.

Referring to FIG. 16 , a plot of the refractive index is shown forsilicon oxynitride films deposited using such a gas mixture in a PECVDsystem. The ordinate in the plot in FIG. 16 shows the flow rate of amolecular precursor gas that contains silicon and the abscissa shows therefractive indices for a measurement wavelength of 633 nm thatcorrespond to specific values of the precursor gas flow rate used in thedeposition of multilayer dielectric stack structures. The plot includessample data that show the variation in a representative opticalproperty, namely the refractive index, for silicon oxynitride films thatwere deposited over a range of gas flows for the silicon-containingprecursor, holding other gas flows and PECVD system parameters constant.The capability to control film parameters such as the refractive index,is reflective of the capability to control the stress level as is madeapparent with the data shown in FIG. 12 b.

The data provided in FIG. 16 , when combined with the measured stressesover a similar range of refractive indices as provided in FIG. 12 b ,demonstrate the benefits of providing the means to control both theoptical properties (refractive index) and mechanical properties (stress)of deposited films with variation of the gas flow mixture in a PECVDdeposition process. The range of gas mixtures shown in the exemplarysample data shown in FIG. 16 , produced with variations in thesilicon-containing precursor gas flow, provide a wide range of measuredrefractive indices for silicon oxynitride films. The variation in themeasured refractive index are shown in FIG. 12 b to produce the lowvalues of residual stress in deposited silicon oxynitride films, andthat a crossover or intersection point of zero stress can be produced ata specific value of the silicon-containing precursor gas flow. Theinnovation provided using the processes described herein should becomeincreasingly apparent in embodiments in which the achievable optical andmechanical properties are further combined with the inventivesuperlattice stack structures described herein for the dielectric stackstructures 700 as described in FIG. 7 . Combinations of refractive indexand film thickness yield stack structures in embodiments that have lowstress and low optical loss when deposited with PECVD processes that donot contain hydrogen in the nitrogen and oxygen precursor gases.

In embodiments of the PECVD processes described herein, a dielectricstack structure 700 is provided wherein the ratios of the precursorgases in the PECVD process are configured to achieve a stress level ofless than approximately 20 MPa in magnitude. In other embodiments, thedielectric stack structure 700 is a structure of silicon oxynitridelayers deposited in a PECVD system using a process that contains silane,nitrogen, and nitrous oxide wherein the ratios of these precursor gasesare configured to achieve a stress level that is less than orapproximately 20 MPa. In embodiments, control of the stress is achievedwith the control of the PECVD gas mixture ratio in combination with thedeposited film thicknesses for each of the individual layers in thedielectric film structure 700, as described in examples provided herein.Alternatively, other PECVD system parameters can be shown to produce asimilar means for controlling the stress in the silicon oxynitridelayers.

In embodiments of the dielectric stack structures 700, for example, thestoichiometric concentrations of the individual layers are selected, asreflected in the measured refractive index of the individual layers inthe stack structure, and optimized with the layer thicknesses to producelow stress film structures with low optical signal loss characteristics.In the formation of the stacked dielectric structure 700, the totaleffective refractive index and the stress level in the film stack 700are affected by the number of layers, the characteristics of theinterfaces between the layers, the stoichiometric concentration, theresulting density of the deposited films, and the presence of impuritiesin the deposited films. This suggests that the measured refractive indexmay not be unique. Although the refractive index provides an effectivemethod for characterizing the effect of changes in process parameterssuch as the gas flow on the optical properties of the depositeddielectric films, multiple stoichiometric concentrations may exist forthe elemental constituents of the silicon oxynitride layer for a givenvalue of refractive index. Nonetheless, the refractive index is aneffective means for providing a measure of changes in the filmproperties, and most notably, a measure of the residual stress indeposited films.

The data shown in FIG. 16 are provided for demonstrative purposes inthat the process parameters such as plasma power, gas pressure,substrate temperature, and gas ratios in the PECVD system can influencethe measured refractive index. The effects of the variation in therefractive index are likely a reflection of the variation in thestoichiometric concentrations and perhaps the density of the depositedfilms. It is important to note, therefore, that the refractive index maynot be unique to a single stoichiometric combination of silicon, oxygen,and nitrogen in the silicon oxynitride layers.

Variations in the embodiments in which the dielectric stack structure700 is formed with the silicon, oxygen, and nitrogen precursors asdescribed herein remain within the scope of the current invention.

In embodiments, the low stress, low optical loss dielectric filmstructures 700 are formed using plasma enhanced chemical vapordeposition with a gas mixture that does not include hydrogen-containingprecursors for either nitrogen, oxygen, or both, or that includesconcentrations of these gases that are low enough so as to not requirehigh temperature processing of greater than 500° C. for example, or highthermal budget processes, to achieve the low optical signal losses ofless than approximately 1 dB/cm in waveguides fabricated from thedielectric stack structure 700. The use of hydrogen-containing, oxygenand nitrogen precursors, such as ammonia, for example, can lead to highlevels of optical loss for deposition processes of less thanapproximately 500 C. In some embodiments, a small amount ofhydrogen-containing nitrogen or oxygen precursor gas, or both, ofapproximately 5-10% or less of the total precursor gas flow, is added tothe PECVD gas mixture. In these embodiments, the concentrations ofhydrogen-containing nitrogen and oxygen precursors could be low enoughso as to not require high thermal budget processes to achieve the lowoptical signal losses of less than 1 dB/cm in waveguides fabricated fromthe dielectric stack structure 700. In yet other embodiments, anitrogen-containing precursor such as nitrogen gas is combined with asmall amount of a hydrogen-containing precursor gas, such as ammonia,for example, and one or more precursors that contain both oxygen andnitrogen, such as nitrous oxide and nitric oxide, and further combinedwith silane or another silicon-containing precursor as described hereinin a plasma enhanced chemical vapor deposition system to form thesilicon oxynitride layers in the dielectric stack 700. In theseembodiments, the amount of ammonia in the process should be sufficientlylow, less than approximately 10% of the total gas flow for example, soas to not require high temperature processing of greater than 500° C.for example, or processing with high thermal budget processes, toachieve the low optical signal losses of less than 1 dB/cm in waveguidesfabricated from the dielectric stack structure 700. Although increasesin the optical signal loss are anticipated, the inclusion of smallamounts of ammonia to the gas mixture can increase the deposition rateof the silicon oxynitride films.

FIG. 12 b shows the variation in the measured stress levels in depositedsilicon oxynitride films over a range of measured refractive index forthese films. The data in FIG. 12 b show that the stress in the depositedfilms using embodiments of the processes described herein can be variedover a wide range between compressive stress and tensile stress with thecrossover at OMPa in the measured stress observed at the transitionbetween the films being in tension and compression. In embodiments, thestress in each layer in the inventive stack structures is varied suchthat the total stress in the dielectric film structure as shown in FIG.7 is less than approximately 20 MPa. The stresses in each of the layersof the inventive stack structure as described in FIG. 7 , is controlledin embodiments by varying the gas ratio, as for example as shown in FIG.12 b , and by varying the corresponding thickness of each of the layerssuch that the total stress in the resultant film stack is less thanapproximately 20 MPa. Examples of combinations of specific refractiveindices, produced with variations in the nitrogen to nitrous oxide gasratio, are described herein. Additionally, the thickness of each layercan be used to compensate for the buildup of stresses in the dielectricfilm stack 700.

In embodiments, when the use of hydrogen-free nitrogen and oxygenprecursors is combined in the deposition chemistry with substratetemperatures during deposition of less than 400° C. in embodiments, andof approximately 300° C. in yet other embodiments, the resulting filmstacks can be produced that exhibit stress levels of less thanapproximately 20 MPa. Additionally, in embodiments, the film stacks aresubsequently patterned to form optical waveguides that provide lowlosses for optical signals propagating within these waveguides. Examplesof embodiments of the deposited dielectric film stacks are describedherein, and in particular, are described in the relevant discussion ofFIG. 7 described herein.

In embodiments, the stack of silicon oxynitride films is formed onto asubstrate using a PECVD process for which the process parameters,including the precursor gas ratios, are configured to achieve a stresslevel of less than 25 MPa, and in preferred embodiments less than 20MPa. Measured stress levels in the deposited film stacks of less than 20MPa greatly reduce the potential for either the substrate to deform, thedeposited films to delaminate, or for some other undesired effect tomanifest in either the dual core waveguide 760 or a substrate,interposer, or subassembly that includes the dual core waveguide 760.Substrate deformation and film delamination are just two forms of damagethat can occur when stresses in the deposited film structures are notadequately controlled. In embodiments, the deposited dielectric filmstacks with stress levels of less than 20 MPa are patterned into one ormore of waveguides and optical devices.

In yet other embodiments, a stack of silicon oxynitride films are formedonto a substrate using a PECVD process for which the gas ratios areconfigured to achieve a stress level of less than 20 MPa and for whichthe process chemistry includes a silicon-containing precursor that mayor may not contain hydrogen, and one or more molecular precursors thatcontain nitrogen or oxygen elements that contains little or no ammoniaor other hydrogen-containing gas. In embodiments, the deposited filmstacks with stress levels of less than 20 MPa that are deposited usingnon-hydrogen-containing nitrogen and oxygen gas sources are patternedinto one or more of waveguides and optical devices.

In yet other embodiments, a stack of silicon oxynitride films are formedonto a substrate using a PECVD process for which the gas ratios areconfigured to achieve a stress level of less than 20 MPa and for whichthe process chemistry does not include ammonia or otherhydrogen-containing gas, other than silane. Silane (SiH₄) is widely usedin industry for the deposition of silicon-containing films and is usedin some embodiments. In embodiments, the deposited film stacks withstress levels of less than 20 MPa, that are deposited usingnon-hydrogen-containing nitrogen and oxygen gas sources such asnitrogen, oxygen, nitric oxide, and nitrous oxide, for example, arepatterned into one or more of waveguides and optical devices.

In embodiments, the gas pressure in the PECVD system during thedeposition of the silicon oxynitride films is in the range of 1 mT to100 Torr to produce dielectric film stack structures. In otherembodiments, process pressures are in the range of 50 mTorr to 2 Torr.In yet other embodiments, process pressures are in the range of 100 to5000 mTorr. The process pressure need not be the same for every step inthe deposition sequence required to produce the full dielectric filmstructure 700.

In embodiments, the total gas flows during deposition of dielectricstack structure 700 is in the range of 5 sccm to 5000 sccm. In someembodiments, the silane gas flow is in the range of 3-1000 sccm. Inother embodiments, the silane gas flow, or other silicon-containingprecursor gas, is in the range of approximately 10-100 sccm.

In embodiments, the nitrogen and oxygen precursor gas flows can varyover a wide range. In some embodiments, the nitrogen and oxygenprecursor gas flows are in the range of 0 to 5000 sccm.

In some embodiments, for which nitrogen is combined with nitrous oxideor nitric oxide, the nitrogen gas flow and nitrous oxide gas flows arein the range of 0 to 5000 sccm. And in some other embodiments, for whichoxygen is combined with nitrous oxide or nitric oxide, the oxygen gasflow and nitrous oxide gas flows are in the range of 0 to 5000 sccm.

In a PECVD system, the input power to the plasma can vary over a widerange. Typical input power for the dielectric stacks in embodiments isapproximately 25-2000 W. In some embodiments, the input power duringplasma enhanced deposition steps is in the range of 200 to 700 W. In yetother embodiments, the input power during plasma enhanced depositionsteps is in the range of 500 to 1200 W. The process power need not bekept constant for the deposition of each of the films in the dielectricstack structure 700 but can vary from step to step, or within adeposition step.

In embodiments, one or more frequencies in the range of 1 kHz to 1 GHzcan be used for the RF power provided to the plasma. In an embodiment, afrequency of 13.56 MHz is used to generate the plasma. In otherembodiments, other frequencies or combinations of frequencies are usedto generate the plasma. In some other embodiments, RF power with afrequency of approximately 27 MHz is applied to the powered electrode.In a preferred embodiment, 13.56 MHz RF power is applied to the upperelectrode with the substrate residing on a grounded electrode during thedeposition of the dielectric film structure 700. In other embodiments,the electrode upon which the substrate resides during the depositionprocess is not grounded. In yet other embodiments, the power is appliedto both the upper and lower electrodes, divided either equally orunequally between the two electrodes.

Power can be delivered to the PECVD system in either capacitive orinductive operational modes and remain within the scope of the currentinvention. For inductive-coupled PECVD systems, the plasma is typicallygenerated with power that is delivered to the plasma via an antenna andthe substrates upon which the films structures 700 are depositedtypically reside on a bottom electrode similar to that shown in thecapacitively coupled configuration shown in FIG. 15 .

Inert gases, such as argon and helium, are added to the gas mixture inone or more steps of the PECVD process in some embodiments, to yielddielectric film stacks with low stress and low optical loss. Inert gasesare frequently added to deposition process chemistries to modify filmproperties, such as the stoichiometry or density of the film. Inembodiments, argon is added to a gas mixture of silane, nitrous oxide,and nitrogen. In yet other embodiments, helium is added to the gasmixture of silane, nitrous oxide, and nitrogen. In yet otherembodiments, one or more of argon and helium is added to a process gasmixture that includes a silicon-containing precursor gas that may or maynot contain hydrogen, a nitrogen precursor that does not includehydrogen in the nitrogen-containing precursor gas molecule or molecules,and an oxygen precursor gas that does not contain hydrogen in theoxygen-containing precursor gas molecule or molecules. Examples ofnitrogen precursors that do not include hydrogen include molecularnitrogen (N₂), nitrous oxide (N₂O), and nitric oxide (NO). Examples ofoxygen precursors that do not include hydrogen include molecular oxygen(O₂), nitrous oxide (N₂O), nitric oxide (NO), carbon monoxide (CO), andcarbon dioxide (CO₂).

Commercially available deposition systems such as the APM PECVD modelmanufactured by SPTS provide advanced programmability for the depositionof the overall dielectric film structure 700 and for each step in thedeposition process. The programmability of commercially availabledeposition systems enables automated operation of the system hardwarethat includes, for example, the mass flow controllers, the source powersupply or supplies, the temperature controllers, and the pressurecontrollers. The process parameters such as, for example, the gas flowrates, the power level or levels, the pressure, the substratetemperature, among other parameters for each step in the deposition of afilm structure can be programmed into a process recipe. Use ofprogrammable and commercially available deposition systems is widelyused in the industry and the use of programmable deposition systems isanticipated and with the scope of the current invention.

In embodiments, the formation of spot size converters in which the dualwaveguide structure described herein is implemented, requires theformation of tapered regions 557, which can be fabricated in a number ofways. In some embodiments, gray scale lithographic patterning techniquesare used with an etch process for forming the thinned portion 555 andthe tapered portion 557 in the upper waveguide core 554. Referring toFIG. 17 , a schematic drawing of key elements of a gray scalelithographic patterning operation is shown. In FIG. 17 , light source1788 a is incident upon gray-scale reticle 1789 to produce aposition-dependent light pattern of varying intensity incident on lightsensitive photoresist layer 1758. The variation in intensity can beachieved, for example, by varying the opacity of the reticle 1789 withposition. Variation in the opacity of the reticle 1789 selectivelyinhibits the transmission of the light 1788 a through the reticle 1789to yield patterned light 1788 b below the reticle 1789. Exposure of thephotoresist layer 1758 changes the properties of this photosensitivelayer 1758 so that the volume of the photoresist 1758 a exposed to thelight source is removable in a suitable developer solution, thus forminga gradient in the mask layer. In the exemplary process shown in FIG. 17, the light exposed regions 1758 a are removed after exposure to aphotoresist developer solution and the shaded region 1758 b remainsafter exposure to the photoresist developer solution (commonly referredto in industry as a “negative” photoresist.)

A three-dimensional depiction of the remaining feature after lightexposure and subsequent exposure to a developer solution to remove thelight exposed regions 1758 b is shown in FIG. 18 a . FIG. 18 a shows theshape of a patterned photoresist mask layer after exposure using a formof gray scale lithography. Other gray scale lithographic techniques areused in other embodiments to produce the tapered mask layer 1758 b shownin FIG. 18 a . In other embodiments, for example, the tapered mask layer1758 b in the photosensitive mask layer 1758 is produced using aphotoresist exposure in which the properties of the light-exposedportions of the photosensitive layer 1758 are such that exposure to thelight sensitizes the layer 1758 to the developer solution (commonlyreferred to in industry as a positive photoresist.) Light exposure isknown to produce cross-linking of the polymer chains in the photoresist.The cross-linking in the photoresist, when combined with a suitabledeveloper solution enables the formation of patterned regions in thephotoresist. The use of gray-scale masking during the exposure of thephotoresist to the lithographic patterning tool enables gradations inthe intensity of the exposure, and hence the depth to which the incidentlight is able to penetrate into the photoresist layer during anexposure. The tapered regions in the photoresist in embodiments are thusformed using the variation in light intensity to which the photoresistis exposed combined with the exposure to the suitable developersolution.

Also shown in FIG. 18 a is the upper waveguide core 1754 and substrate1711. The upper waveguide core 1754 is comprised of the layer or layersas described herein for upper waveguide core 754 of film structure 700.In the embodiment shown in FIG. 18 , the structure does not include anupper spacer layer 758. In some embodiments in which a gray scalelithographic technique is used to form the tapered portion of the upperwaveguide core, an upper spacer layer is present between thephotosensitive layer 1758 and the upper waveguide core 1754. Substrate1711, as shown in FIG. 18 , includes all of the layers below the upperwaveguide core 754 (including layers 730, 738, 742, 750, if present) andincluding the substrate 710, as described herein, for example, for thestructure 700. In the embodiment depicted in FIG. 18 , the upperwaveguide core 1754 is patterned using a gray scale lithographictechnique.

Referring to FIG. 18 b , the schematic drawing shows the taperedphotoresist from FIG. 18 a after exposure to an etch process to form thetapered features, as well as untapered features, in the upper waveguidecore 1754. In portions of the structure in which the untapered masklayer remains, the full thickness of the upper waveguide core 1754remains. In the areas of the structure in which the waveguide layer 1754has been exposed to the etch process, a thinner layer 1755 remains asshown. In embodiments, the substrate 1711 includes substrate 710 and allof the layers above the substrate 710 and below the upper waveguide core754 including buffer layers, spacer layers, and lower waveguide core asdescribed herein. In some embodiments that utilize silicon oxynitridelayers in the upper waveguide core 1754, plasma processing is used withfluorinated gas chemistries to etch the silicon oxynitride. Combinationsof carbon tetrafluoride (CF₄), trifluoromethane (CHF₃), sulfurhexafluoride (SF₆), octafluorocyclobutane (C₄F₈), Argon (Ar), oxygen(O₂), among others, are commonly used in the art for patterning ofsilicon oxide, silicon nitride, and silicon oxynitride films and areused in embodiments in combination with gray-scale lithography toproduce the etched structure shown in FIG. 18 b . Plasma processmodules, or plasma reactors as they are sometimes referred, can becapacitively coupled, inductively coupled, or microwave sources. Thecapacitively-coupled modules can be parallel plate modules with singleor multiple frequency excitation. Dry etching of dielectric materialssuch as silicon oxide, silicon nitride, and silicon oxynitride films arewell known in the art. In embodiments, the plasma etch process iscombined with gray scale lithographically patterned photoresist toproduce the tapered structure features in the upper waveguide core 1754as shown in FIG. 18 b . Exposure of the thinnest end of the taperedregion (left side as shown) in the mask layer 1758 b, in embodiments,causes the mask layer to recede during the etch which increases theexposure of the area below the mask as the mask recedes to produce thetaper in the upper waveguide core layer 1754. It is important to notethat the tapered portion of the structure shown in FIG. 18 is theportion of the embodiment that generally requires and benefits from theuse of gray scale lithographic patterning. Production of the vertical ornear vertical sidewalls does not require gray scale techniques. It isalso important to note that the vertical or near-vertical faces shown atthe right of the patterned feature 1758 b in FIG. 17 and FIG. 18 can beextended to produce intact or substantially intact upper waveguide coresfor the routing of optical signals as required by the optical device.Referring, for example, to FIG. 5 and FIG. 6 , the non-tapered portionsof the patterned feature 1758 b can be used to produce a patternedwaveguide structure such as that required to produce an arrayedwaveguide. In other embodiments, all or part of other optical devicesthat require tapered features can use gray scale lithographic techniquesas described in FIG. 17 and FIG. 18 , for example, to form the taperedsections.

Referring to FIG. 18 c , the upper waveguide core is shown after theetch and the removal of any remaining mask layer that may have remainedafter the etch exposure. FIG. 18 c shows the thinned areas 1755 of theupper waveguide core and the tapered regions 1757 of the upper waveguidecore 1754. In the embodiment depicted in FIG. 18 c , the thinnedportions 1755 are shown as they might appear after the tapered featurewith an etch that produces the vertical faces on the sides of the upperwaveguide core 1754. In some embodiments, the etch that produces thetaper is separate from the etch that produces the vertical faces. Theexemplary embodiment of the gray scale lithographic process is intendedto show a method to produce the tapered section 1757. In otherembodiments, multiple patterning steps may be used to produce thespecific features required for the optical waveguide structure such asthe tapered feature 1757 and the vertical faces of the upper waveguidecore 1754.

In FIG. 18 d , an embodiment is shown in which the upper waveguide corehas been subsequently repatterned and exposed to an etch process topattern the thin section 1755 of the upper waveguide core 1754. In thisfigure, the upper waveguide core 1754 is shown after patterning andetching step to form a resulting dual core waveguide structure similarto the structures shown in FIG. 4 c.

The exemplary gray scale lithographic method described herein is anexample of a sequence of steps that are used to form the thinnedsections 555 and the tapered sections 557 in the upper waveguide core1754 of a dual core waveguide, as might be used in the formation of aspot size converter or other optical device structure for use in anoptical or optoelectronic circuit. Other embodiments using gray scalelithography are anticipated and within the scope of the currentinvention.

Other standard lithographic pattering techniques without gray scalepatterning are used in other embodiments. In an embodiment, standardlithographic patterning is combined with an aspect ratio dependent etchprocess to form the thinned portion 555 and the tapered portion 557 inthe upper waveguide core 554. In FIG. 19 , an embodiment is shown thatillustrates the effect that the combination of a lithographic patterningstep with an aspect dependent etch process has on the formation of atapered portion of a dual core waveguide. Referring to FIG. 19 a , amasked area 1961 is exposed to a light source through an optical reticleto produce the pattern shown in the top view of FIG. 19 a . The darkershaded area 1954 in FIG. 19 a is the exposed upper waveguide core layer1954 within the cavity in the photoresist layer that has been formed,for example in an embodiment, upon exposure to a developer solution toremove the light-exposed resist. Upon exposure to the developersolution, the cavity is formed and underlying layer 1954 is revealed.The cross-section detail A-A′ in FIG. 19 a shows an embodiment of astack structure comprised of the substrate 1910, the lower core 1942 ofthe dual core waveguide and the upper waveguide core 1954 with theopened mask layer 1961 and the exposed surface of the underlying layer1954. The underlying layer 1954 is an upper core of a dual corewaveguide 760. In embodiments, other layers are present between the masklayer 1961 and the upper core 1954 of the dual core waveguide asdescribed herein, for example, for the structure 700 shown in FIG. 7 .Mask layer 1961, in some embodiments, is a combined structure of aphotosensitive photoresist layer and any components of the overall dualcore waveguide structure 700 that reside between the photosensitive masklayer and the upper waveguide core layer 1954. In some embodiments, themask layer 1961 includes a hard mask layer. In other embodiments, themask layer 1961 includes one or more of a photoresist layer, a hard masklayer, and capping layer 758. In other embodiments, the layer 1961includes a portion of upper waveguide core 1954. In embodiments in whichthe mask layer 1961 includes layers other than photoresist, suitableetch processes are used to remove these layers within the cavity toexpose upper waveguide core layer 1954 as shown schematically in the topview and the Section A-A′ views in FIG. 19 a.

Referring to the top view shown in FIG. 19 b , an embodiment of the dualcore waveguide structure 1954 is shown after an etch process to patternthe upper waveguide core 1954 of the dual core waveguide structure, andto form thinned upper waveguide core section 1955 and tapered upperwaveguide core 1957. The etched areas of the upper waveguide core 1954are indicated. In an embodiment, the upper waveguide core 1954 isexposed to a plasma etch process with process chemistry and conditionssuch that the vertical etch rate, or the etch depth, is dependent on thewidth of the opening. These types of processes are commonly referred toas aspect ratio dependent etch processes and understood by those skilledin the art of thin film etch processing. Plasma etch process conditionsare employed in embodiments, so as to produce the highest etch rate inthe widest region of the etch area (at the leftmost edge). As the openarea becomes narrower, following along the horizontal section of lineB-B′ from left to right, the etch rate or etch depth of the dualwaveguide structure using the applicable etch process conditionsdecreases. The decreasing etch rate with opening width, known in theindustry as an aspect ratio dependent etch process, is used in someembodiments, to produce the tapered section 1957 as shown in thecross-sectional view in FIG. 19 b . Also shown in the cross-section inFIG. 19 b is the partially etched portion 1955 of the upper waveguidecore 1954. As is the case with the embodiments illustrated in FIG. 18 ,the embodiment shown in FIG. 19 can be used to produce the taperedportion 1957 of an upper waveguide core 1954 as is used for example in aspot size converter or other optical device or part of an opticaldevice. Multiple structures as shown in FIG. 18 and FIG. 19 can becombined to produce a device or combination of devices using theinventive planar dual core waveguide 700.

In some embodiments, the present invention discloses an optical devicefor processing an optical signal 502 from an optical fiber 580. Theoptical device can be configured to be directly coupled to the core 582of the optical fiber 580 using a dual waveguide configuration. The dualwaveguide can include a first waveguide 542 having a thicknesscomparable with a thickness of the core 582 of the optical fiber 580.Thus the first waveguide 542 can be directly coupled with the core 582of the optical fiber 580 due to the thickness matching. For example, thecore 580 of the optical fiber 580 can have a thickness between 5 and 10microns for single mode optical signal propagation. The first waveguide542 can have a matching thickness, for example the first waveguide 542can also have a thickness between 5 and 10 microns. The matchingthickness can allow the optical signal 502 from the core 582 of theoptical fiber 580 to be transmitted to the first waveguide 542.

The dual waveguide 560 can include a second waveguide 554 disposedadjacent to the first waveguide along a length of the first waveguide542. For example, the second waveguide 554 can be on top or under thefirst waveguide 542. The second waveguide 554 can include a firstportion 555 which is configured to be weakly coupled to the firstwaveguide 542 to share the optical signal 502 in the first waveguide 542that is transmitted from the optical fiber 580. The length of the firstportion 555 can be configured to optimize an optical signal propagationbetween the first waveguide 542 and the second waveguide 554. The firstportion 555 can have an index of refraction greater than an index ofrefraction of the first waveguide 542, for example, to facilitate thecoupling between the second portion of the second waveguide 554 and thefirst waveguide 542, for example, to promote a signal propagation fromthe first waveguide 542 to the first portion 555 of the second waveguide554.

The second waveguide 554 can include a second portion which isconfigured to further transmit the optical signal 502 partly received inthe first portion 555. The second portion of the second waveguide 554can be configured to be decoupled from the first waveguide 542, forexample, there is no signal sharing between the second portion of thesecond waveguide 554 and the first waveguide 542.

The second waveguide 554 can have multiple thicknesses, such as one ormore thicknesses for the first portion 555, and a thickness for thesecond portion. The thicknesses of the second waveguide 554 can besmaller than that of the first waveguide 542. Since the first waveguide542 has a large thickness, for example, a thickness having comparabledimension with the core 582 of the optical fiber 580, optical signalpropagation in the first waveguide 542 can be susceptible to multimodepropagation, for example, there can be undesirable optical modesgenerated in the first waveguide 542, even with the single mode opticalsignal coming from the single mode optical fiber 580. Thus, thethicknesses of the second waveguide 554 can be optimized for single modesignal propagation, such as configured to promote single mode signalpropagation with a thickness equal or less than 3 microns.

The first portion 555 of the second waveguide 554 can have one or morethicknesses. The thicknesses of the first portion 555 can be configuredso that the first portion 555 is coupled, for example, weakly coupled,to the first waveguide 542, such as by having thicknesses equal or lessthan about 3 microns, less than 1 micron, or less than 0.5 micron.

The thickness of the second portion of the second waveguide 554 can beconfigured so that the second portion is decoupled from the firstwaveguide 542, such as by having a thickness greater than about 0.5 or 1micron.

The optical device can be configured to receive an optical signal 502from an optical fiber 580, or can be configured to send an opticalsignal 502 to an optical fiber 580. For example, an optical signal 502can be sent from the optical fiber 580 to the first waveguide 542,through the weakly coupling portions of the second waveguide 555, 557,and transferred substantially to the second portion of the secondwaveguide 554. Alternatively, an optical signal can be sent from thesecond waveguide 554, through the weakly coupling portions of the secondwaveguide 555, 557, transferred to the first waveguide 542, and then tothe optical fiber 580.

In some embodiments, a waveguide, such as the first waveguide 542 orsecond waveguide 554, can include one or more layers configured foroptical signal propagation. The waveguide can further include otherlayer, such as buffer layers or cladding layers, which can be configuredto confine the signal in the propagation core. Other layers can beincluded, depending on the needs of the waveguide. For example, a layercan be disposed between the first and second waveguides, which isconfigured to improve the weak coupling between the first and secondwaveguides.

In some embodiments, the first portion of the second waveguide 554 caninclude an adiabatic transition section 557, which is configured totransition the second waveguide 554 from a thickness or range ofthicknesses that allows coupling with the first waveguide 542 to thesecond portion, which has a thickness that provides a decoupling withthe first waveguide 542. The adiabatic transition section 557 can have atapered thickness from a thinner thickness, for example, a thicknessthat allows coupling with the first waveguide 542, to a thickerthickness, for example, a thickness that does not provide coupling withthe first waveguide 542, which is the thickness of the second portion ofthe second waveguide 554.

The first portion of the second waveguide 554 can include only anadiabatic transition section 557, for example, a tapered sectionconnected to a thicker second portion of second waveguide 554.Alternatively, the second waveguide 554 can include a first portion thatcan include a flat portion 555 and an adiabatic transition section 557,for example, the second waveguide 554 can include a first portion 555that can have a first part having a flat thickness, which is thencoupled to a second part having a tapered section connected to thesecond portion. Thus, the flat portion can have a zero length in thecase of a first portion that only includes the adiabatic transitionsection 557. The flat portion 555 and the adiabatic portion 557 can beconfigured to optimize signal propagation from the first waveguide 542to the second portion of the second waveguide 554.

In some embodiments, the first waveguide 542 can be formed by depositinga waveguide layer on a substrate. Other layers can be included, such asa buffer layer under the waveguide layer. The second waveguide 554 canbe formed by depositing a thinner waveguide on the first waveguide 542.Other layers can be included, such as a layer disposed between the firstand second layer for optimizing the signal coupling between the firstwaveguide 542 and the second waveguide 554. After depositing the thinwaveguide layer, the thin waveguide layer can be patterned to form thesecond portion of the second waveguide 554. The patterning process canform the second portion, together with forming an outline for the firstportion 555,557. For example, the patterning process can form an outlineof the second waveguide 554, for example, removing lateral portions ofthe thin waveguide layer outside the outline of the first and secondportions. The lateral patterning can form the second portion of thesecond waveguide 554, and the lateral shape of the first portion 555,557, with the first portion still needing vertical patterning. Thelateral shape of the first portion can then be patterned to form thefirst portion. For example, the flat portion 555 of the first portion,if there is one, can be etched vertically to achieve an appropriatethickness.

In some embodiments, the dual core waveguide structure can be patternedusing a gray scale lithography process or an aspect ratio dependent etchprocess, especially for the adiabatic transition section 557. The flatportion 555 and the second portion of the second waveguide 554 can havea flat top surface, which can be patterned by an etch process using aconventional lithography mask, for example, a mask that allows uniformetching on exposed areas. The adiabatic transition section 557 can havea tapered surface, which can be patterned by an etch process using agray scale lithography process or an aspect ratio dependent etchprocess, for example. The gray scale lithography mask or the aspectratio dependent mask can also include a portion of the conventionalmask, in order to etch both the adiabatic transition section 557, theflat portion 555, and the second portion of the second waveguide 554 ata same time using a same mask.

In addition to the thickness taper in the adiabatic transition section557, the first portion can have a horizontal taper, for example, toallow a low loss transition from a width comparable with the dimensionof the core 582 of the optical fiber 580 to a smaller width optimizedfor single mode signal propagation in the second portion of the secondwaveguide 554. The first waveguide 542 is configured to be coupled withthe optical fiber 580, with a thickness and a width having a comparabledimension with the core 582 of the optical fiber 580. Thus, at thelocation near the coupling area between the core 582 of the opticalfiber 580 and the first waveguide 542, the width of the second waveguide554 can be similar to that of the first waveguide 542. The width of thesecond waveguide 554 is then tapered in the width direction, such astapered in the flat portion 555 and in the adiabatic transition section557. The horizontal taper can be a smooth taper, or can be a piecewisetaper, such as a first taper in the flat portion 555 and a second taperin the adiabatic transition section 557.

In some embodiments, the first waveguide can include a repeated stack oftwo or more SiON layers on a buffer layer. The two or more layers canhave at least two layers having different indexes of refraction. Thefirst waveguide 542 can include a repeated stack of two or more SiONlayers. The two or more layers can include layers having differentindexes of refraction. Each layer of the buffer layer and the layers ofthe repeated stack comprises a stoichiometry of Si, 0, and N to providea stress having a magnitude less than or equal to 20 MPa. The firstwaveguide 542 can include a repeated stack of two or more SiON layers.The two or more layers can include layers having different indexes ofrefraction. Each layer of the buffer layer and the layers of therepeated stack comprises a level of impurity or a level of homogeneityto provide an optical loss less than or equal to 1 dB/cm.

In some embodiments, the present invention discloses an optical devicefor coupling an optical signal from an optical fiber 580 to a photonicintegrated circuit, including an optoelectronic device. The opticaldevice can be configured to be directly coupled to core of the opticalfiber 580 using a dual waveguide configuration, having a first waveguide542 coupled to a second waveguide 554. The output of the secondwaveguide 554 can be coupled, directly or indirectly through anintermediate component, to the photonic integrated circuit.

The optical device can be configured to receive an optical signal 502from an optical fiber 580, or can be configured to send an opticalsignal 502 to an optical fiber 580. For example, an optical signal 502can be sent from the optical fiber 580 to the first waveguide 542,through the weakly coupling portions 555,557 of the second waveguide554, and transferred to the second portion of the second waveguide 554,and then to the photonic integrated circuit. Alternatively, an opticalsignal 502 can be sent from the photonic integrated circuit to thesecond waveguide 554, through the weakly coupling portions 555, 557 ofthe second waveguide 554, and transferred to the first waveguide 542 andthen to the core 582 of the optical fiber 580.

The optical signal 502 can be processed in the optical device, forexample, at the second portion of the second waveguide 554, which iscoupled to the first portion. The second portion of the second waveguide554 can include an array waveguide, a grating, a filter, a blocker, aprism, a combiner, a multiplexer, a de-multiplexer, a splitter, anechelle grating, or a combination thereof. Thus, an optical signal 502can be transmitted from the optical fiber 580, and then processed at thesecond portion of the second waveguide 554 of the optical device, beforesending to the photonic integrated circuit.

In some embodiments, the present invention discloses an optical devicefor coupling different optical fibers, for example, from one or morefirst optical fibers 580 to one or more second optical fibers 580. Theoptical device can be configured to be directly coupled to the opticalfibers 580 using a dual waveguide configuration. The dual waveguide caninclude a first portion 555,557 of a second waveguide 554 weakly coupledto a first part of a first waveguide 542, which is directly coupled toone or more first optical fibers 580. The dual waveguide can include asecond portion of the second waveguide 554 weakly coupled to a secondpart of the first waveguide 542, which is directly coupled to one ormore second optical fibers 580.

The optical device can be configured to convey an optical signal 502between one or more first optical fibers 580 to one or more secondoptical fibers 580. For example, one or more optical signals 502 can besent from the one or more first optical fibers 580 to the firstwaveguide 542, through a first weakly coupling portion 555,557 of thesecond waveguide 554, transmitted to the second waveguide 554, and thenthrough the second waveguide 554 back to the first waveguide 542 througha second weakly coupling portion 557,555, and then to the one or moresecond optical fibers 580. Alternatively, one or more optical signals502 can be sent from the one or more second optical fibers 580 to thefirst waveguide 542, through a first weakly coupling portion 555,557 ofthe second waveguide 554, transferred to the second portion of thesecond waveguide 554, through the second waveguide 554, back to thefirst waveguide 542 through a second weakly coupling portion 557,555,and then to the one or more first optical fibers 580.

The optical signal 502 can be processed in the optical device, forexample, at the second portion of the second waveguide 554, which iscoupled to the first portion 555,557. The second portion of the secondwaveguide 554 can include an array waveguide, a grating, a filter, ablocker, a prism, a combiner, a multiplexer, a de-multiplexer, asplitter, an echelle grating, or a combination thereof. Thus, opticalsignals 502 can be transferred from optical fibers 580, and thenprocessed at the second portion of the second waveguide of the opticaldevice, before sending to the other optical fibers.

The forgoing description of embodiments is provided for the purposes ofillustration, but this description is not intended to be exhaustive orto limit the invention to the precise forms disclosed. Manymodifications and variations will be apparent to a practitioner skilledin the art. Embodiments were chosen and described in order to bestillustrate the principles of the invention and its practicalapplication, thereby enabling others skilled in the art to understandthe invention. The various embodiments described have been presented byway of example, and not limitation. It will be apparent to personsskilled in the relevant art that various changes in form and detail canbe made therein without departing from the spirit and scope of theinvention. The breadth and scope of the invention should not be limitedby any of the above-described exemplary embodiments, but rather shouldbe defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A method comprising forming a first waveguide ona substrate, wherein forming the first waveguide comprises forming arepeated stack of two or more SiON layers on a buffer layer, wherein thetwo or more layers comprise at least two layers having different indexesof refraction, wherein the first waveguide has a first thicknesscomparable with a thickness of a core of an optical fiber and configuredfor directly coupling with the optical fiber; forming a secondwaveguide, wherein the second waveguide interfaces with the firstwaveguide along a length of the first waveguide, wherein the interfacebetween the first and second waveguides comprises a flat surface,wherein the second waveguide comprises a first portion configured to beoptically communicatable with the first waveguide, wherein opticallycommunicatable comprises a capability of optical signal transfer betweenthe first portion and the first waveguide, wherein the second waveguidecomprises a second portion coupled to the first portion.
 2. A method asin claim 1, further comprising coupling the optical fiber to the firstwaveguide, so that either an optical signal from the optical fiber isconfigured to transmit to the second waveguide through the firstwaveguide, or an optical signal from the second waveguide is configuredto transmit to the optical fiber through the first waveguide.
 3. Amethod as in claim 1, wherein the first portion comprises a flat sectionparallel to the flat surface, wherein the flat section is configured toprovide the optical signal transfer capability between the first portionand the first waveguide, wherein the optical signal transfer capabilitycomprises a portion of an optical signal propagating in the firstwaveguide to transfer to the first portion or to a portion of an opticalsignal propagating in the first portion to transfer to the firstwaveguide.
 4. A method as in claim 1, wherein the first portioncomprises a tapered thickness configured for improving optical signalpropagation to the second portion, wherein the second portion isconfigured for single mode optical signal propagation.
 5. A method as inclaim 1, wherein the first portion comprises a tapered thickness formedby patterning a layer using a gray scale lithography process or anaspect ratio dependent etch process.
 6. A method as in claim 1, whereinthe first portion comprises a flat section coupled to a tapered section,wherein the tapered section is configured to provide an adiabatictransition from the first portion to the second portion.
 7. A method asin claim 1, further comprising forming a photonic integrated circuitcomprising electrical contacts, wherein the photonic integrated circuitis directly or indirectly coupled to the second portion.
 8. A method asin claim 1, further comprising forming an optoelectronic device coupledto the second portion, wherein the optoelectronic device compriseselectrical contacts configured to be electrically connected to anelectrical device.
 9. A method as in claim 1, further comprising formingan optical element, wherein the optical element comprises an arraywaveguide, a grating, a filter, a blocker, a prism, a combiner, amultiplexer, a de-multiplexer, a splitter, an echelle grating, or acombination thereof, wherein the optical element is directly orindirectly coupled to the second portion or wherein the second portionis a part of the optical element.
 10. A method as in claim 1, furthercomprising forming a layer between the first portion and the firstwaveguide, wherein the layer is configured to improve the optical signaltransfer capability between the first portion and the first waveguide.11. A method as in claim 1, wherein each layer of the repeated stackcomprises a stoichiometry of Si, O, and N to provide a stress having amagnitude less than or equal to 20 MPa.
 12. A method as in claim 1,wherein each layer of the repeated stack comprises a level of impurityor a level of homogeneity to provide an optical loss less than or equalto 1 dB/cm.
 13. A method as in claim 1, wherein the first portioncomprises a horizontal tapered section.
 14. A method as in claim 1,further comprising forming an optical signal processing device coupledto the second section, wherein the optical signal processing device isconfigured to process the optical signal propagating from or to thefirst waveguide.
 15. A method as in claim 1, wherein forming the secondwaveguide comprises depositing a layer, patterning the layer to form thesecond portion, patterning a portion of the layer to form the firstportion.
 16. A method comprising forming a first waveguide, whereinforming the first waveguide comprises forming a repeated stack of two ormore SiON layers on a buffer layer, wherein the two or more layerscomprise at least two layers having different indexes of refraction,wherein the first waveguide comprises a first thickness comparable witha thickness of a core of an optical fiber, wherein the first waveguideis configured for directly coupling with the optical fiber; forming oneor more second waveguides, wherein at least a portion of each secondwaveguide is interfaced with the first waveguide along a length of thefirst waveguide, wherein the interface between the first waveguide andthe each second waveguide comprises a flat surface, wherein the eachsecond waveguide comprises a first portion configured to be opticallycommunicatable with the first waveguide, wherein opticallycommunicatable comprises a capability of optical signal transfer betweenthe first portion and the first waveguide, wherein the each secondwaveguide comprises a second portion configured to propagate an opticalsignal from or to the first waveguide.
 17. A method as in claim 16,wherein there are more than one second waveguides configured to form anarray of waveguides.
 18. A method as in claim 16, wherein there are morethan one second waveguides configured to form a multiplexer or ademultiplexer with the first waveguide to split an optical signalpropagating in the first waveguide.
 19. A method comprising forming afirst waveguide on a substrate, wherein forming the first waveguidecomprises forming a repeated stack of two or more SiON layers on abuffer layer, wherein the two or more layers comprise at least twolayers having different indexes of refraction, wherein the firstwaveguide has a first thickness comparable with a thickness of a core ofan optical fiber; forming one or more second waveguides, wherein atleast a portion of each second waveguide is interfaced with the firstwaveguide along a length of the first waveguide, wherein the interfacebetween the first waveguide and the each second waveguide comprises aflat surface, wherein the each second waveguide comprises a firstportion configured to be optically communicatable with the firstwaveguide, wherein optically communicatable comprises a capability ofoptical signal transfer between the first portion and the firstwaveguide, wherein the each second waveguide comprises a second portioncoupled to the first portion; providing a first device, wherein thefirst device is coupled to the second waveguide, wherein the firstdevice comprises electrical contacts for connections to a second device.20. A method as in claim 19, wherein each layer of the repeated stackcomprises a stoichiometry of Si, O, and N to provide a stress having amagnitude less than or equal to 20 MPa.